A/N (Edited on 15/11): I received some feedback on Ao3 about making the action scenes a bit crisper and tighter, so I've made those changes here (thanks, nockout!) Also, something I forgot to mention was a trigger warning for some minor gory bits. If you have a sensitive stomach, I'd skip the part where Harry and Ron enter the basement. I'll summarise that section in the end notes.

X-XXX-X

April 2004

As it transpired, Dennis would not be getting the answer to the half-formed question that had only struck him days and weeks after the St. Mungo's incident for quite a while, but then again, the Riggs Moor case took up so much of his mental bandwidth he quite forgot the question in the first place.

He had a pretty even split between stakeouts and research again this week, the way he'd had a few weeks in following the Goyle arrest. Terry had managed to convince a Knockturn apothecary runner into passing them information on a possible Death Eater hideout, and he'd been one of the Junior Aurors tasked with following the man to see if there was any useful information to be found that he hadn't seen fit to share. Unfortunately, the ingredients themselves were a dead end – some of them were restricted or banned substances, of course, that was the hold Terry had, after all; but nothing jumped out at them as particularly egregious. All Savage, their in-house Potions Master, could say was that some of those supplies were dangerous and could be used to manufacture a host of nasty things. Worse, the recipients of the dodgy ingredients lived under the Fidelius, so there was no way for them to get the address and raid the place – or even know how many people were there and who they were. Dennis had seen the evidence with his own two eyes – a disembodied hand emerging from thin air to accept a parcel from the Knockturn bloke, whose own Apparition records showed nothing but blankness.

The Fidelius was the absolute bane of their existence, he thought grumpily. It was unfortunate that the properties that were enchanted before all the new laws were passed and all the tracking and permissions enabled, still retained the protection of the charm. He didn't want to think about how many Death Eaters hadn't been found because they had old, forgotten hidey-holes, all invisible to the Auror Office. Essentially, he was looking in a bunch of files for a piece of information that didn't exist, hoping for someone to slip up.

He was the only Auror who knew a Spellcrafter junior enough to come give them all a discourse on the Fidelius in words they may have actually understood, and so he'd been permitted to ask Orla to come assist. Initially, it had surprised them all that there hadn't been more people protecting themselves with a Secret Keeper, especially before all the newly-minted legislation providing greater Ministry oversight over the process, but that had lasted only until they'd had Orla explain to them exactly how the charm worked.

"The Fidelius Charm is more an expression of faith and belief than anything else. This is what makes it so difficult to cast successfully. For one, the secret has to be entrusted to another, the vulnerability concealed within another person. You have to be able to trust that person with your life, and they must live out of your reach, out of your control. Leaving and re-entering the confines of the charm too often is… inadvisable; if you are the primary residents to be protected - each exit and re-entry introduces weaknesses into the net of the charm. It's like a cocoon, do you see? It is okay for your guests, as protecting them is not the primary goal of the charm. Also, casting Fidelius upon a well-known property is dangerous, the strength of the Charm derives also from the secret's obscurity. Magic leaves traces, and it will leave those traces when it steals the knowledge from all who possess it prior to the charm being cast. It's why you could never put, say, the Three Broomsticks under a Fidelius Charm, even if you wanted to. You would remember the sign, and the general area, and the look and feel of the place. You would remember Madam Rosmerta, and the Butterbeer tankards, and the knowledge of the gap in your memory and the fuzziness around all the public information on the Three Broomsticks would allow you to eventually break the charm, even when the charm is strongest – which is when the original Secret Keeper still lives."

"If he or she dies, all bets would be off for a commonly known location like the Three Broomsticks. Even for a more unknown property, the death of the Secret Keeper risks the charm, unless one is supremely diligent. This is because when the Keeper dies, then all the others who had the secret shared with them become Secret Keepers in turn. It is highly inadvisable for the primary resident of a property to also be its Secret Keeper, by the way – that weakens the charm, especially over time, and it won't stand up to heavy assault – though breaking the Charm would take time, and your Billywigs would undoubtedly fly the nest. I doubt that is the case here anyway, and if it is, my guess would be that the property has been under Fidelius for less than a year, maximum."

"Is there no way for the Secret Keeper to live indefinitely inside the house? We, uh, have experience with that, so how does that work?" Ron asked, leaning across the table.

Orla stopped, adjusting her glasses higher up on her nose. Her expression turned contemplative.

"Usually, that is how it works, because of the limitations of the charm. But I suppose… one way it could be done differently is if the original Secret Keeper is dead – the new Keepers could choose to live inside the dwelling, and the fading of the charm would be much more gradual… I don't actually know if that would work, but it seems like my best guess. Is that how it happened with you?"

"Yes," said Harry shortly, unwilling to get into their time spent at Grimmauld Place after Dumbledore's death. Perhaps sensing his best mate's mood, Ron cut in.

"I just don't see Death Eaters trusting someone that much that they'd let them walk around, though." He sat back, a frown tugging at his brows.

"Could the people being protected – could they entrust the Secret to a Keeper, and then murder them to ensure their silence? Would that be a workaround?" It oughtn't to have been a surprise that that suggestion came from Neville, the only person who really understood how the Room of Requirement back at Hogwarts worked. Dennis wondered briefly why he hadn't gone into law with a brain for loopholes like that.

Orla turned to face a rapidly pinkening Neville, eyes animated. "That's an excellent suggestion – I never thought of that before! Of course, the primary residents of the property can't actually be the ones to choose to betray the Keeper – that would be a violation of the basic tenet of the charm, to entrust. But another could. I would theorise – and this is very much just a theory, mind – that even if the primary protected entrust the secret to the original Keeper purely on the strength of another person - let's call that person X's - recommendation, and even if they suspect that X may resort to some underhanded ways of ensuring the Secret remains protected, such as indefinitely detaining the Keeper, it would not impact the charm as long as they don't know for sure what lengths X might go to."

She ran a hand absent-mindedly through her hair. "It comes down to intent. Do the primary protected intend to entrust the Secret to the Keeper themselves? Then the actions of another need not impact the strength of the charm, as long as they are unaware of any betrayal that might take place later. The murderer, if they were in possession of the secret before the original Keeper was killed, would become a Keeper in turn, and, I hypothesise, they would be able to live in that property for a great deal longer than they would have if they were the original Secret Keeper. I would probably need to consult with more senior colleagues to give you a firm answer, but…"

"That's all very well and good, but how does this help us, exactly?" interrupted Terry, looking impatient. "If they're Death Eaters, they've murdered countless people and one more isn't making much of a difference, is there?"

"A charm with multiple keepers like that – that is vulnerable. If you are able to touch any part of the person who is collecting the package from the courier as they bring themselves fully within the confines of the charm again – even grab the edge of a trailing scarf, as long as it is touching the Keeper, and use it to feel your way in, that'd be enough. But you'll need to be quick – there's no way of gauging what other spells are up, how protected the actual property is. There might be an army on the other side laughing directly in your face and you won't be able to hear it until you are inside the charm."

"How do we know if the original Keeper is alive or not?" asked Harry.

"You don't. It's a risk you have to take – but if you can't make it through, then you know there's a single Keeper – and it's not the person picking up packages."

X-XXX-X

The atmosphere in the Office was tense, tenser than usual before an operation. They'd run drills practicing this scenario for two weeks on end, acutely aware that a single misstep could prove to be unbelievably costly. Terry's informant had told them he was due there at 4pm, and a non-insignificant portion of the plan rested on him.

It all went off swimmingly to start with, to Dennis's (pleased) shock, but he didn't have time to really think about it, because it all seemed to happen in around five seconds. It seemed that Neville's hunch had been correct; because their best-case scenario came to fruition. Either that, or as unlikely as it seemed, the Death Eaters really were foolish enough to send their one and only Secret Keeper to retrieve packages.

The plan was simple – Harry, as the fastest shot, clad in his invisibility cloak, with a Feather-Light charm on himself, would hold on to the barest edge of the parcel, hoping the force of the tug on the parcel would be enough to yank him over the edge of the Charm if the recipient was a Keeper, but that in case he was stopped up short by the Charm, the man on the other side would not notice any resistance. If he made it over the edge, he would Stupefy the yet unidentified recipient, cancel the weight-reducing charm and toss a length of rope attached around his waist out to help the others in. The informant had been told to Disapparate as soon as he handed over the parcel, which he did with alacrity.

That part went off fine, better than fine, but as a disillusioned Dennis broke into the charm, he was jarred by the utter chaos erupting suddenly in his ears. Firstly, his disillusionment charm broke within seconds – there must have been some sort of protective spell over the place that disallowed concealment, because all the Aurors were suddenly visible. A loud klaxon was blaring, and he felt an Anti-Apparition jinx settle into his bones. Secondly, he found his hands full almost immediately, battling an unremarkable, blank faced man; one of many dressed in plain black robes. Spells were flying out of the first storey of the house as well, and out of the corner of his eye he saw Harry, tearing off the cloak, racing towards the house with Ron, fending off spells with incredible agility. The other, more senior Aurors were following them as fast as they could, and it fell to him and the other Juniors and lower ranking Aurors to mop up the mess outside.

The outside combatants were soon out-matched, stupefied and bound – it seemed clear that they were the rank and file, perhaps even enchanted or controlled in some fashion. Once the tide turned in their favour, some of the Juniors ran towards the house as well. Leslie and Rose Zeller stayed outside with a couple of the remaining Junior Aurors once only a few fighters were needed to secure the area, and the moment he had dispatched the two combatants he'd been battling, Dennis found himself racing after his fellow Juniors towards the sounds of spellfire and yelling inside the house, adrenaline thumping so loudly in his ears his heartbeat was almost the only thing he could hear.

Afterwards, thinking back, it was all a blur. Bursting through the doorway, a bludgeoning charm exploding just above his head. Williamson, ashen and bleeding from the throat on the floor, with Maisy providing triage under cover of a shield held by a grim-faced Owen. Loretta Cornhill, unconscious and splayed out on the ground next to her. Harry and Neville battling a snarling, unhinged man that Dennis realised with a sick jolt was Rodolphus Lestrange. Pritchard, one of the strongest duellers amongst the Junior Aurors, working on subduing Corban Yaxley along with Terry. Maisy and Owen levitating Williamson and Cornhill outside of the house, out of harm's way. Ron battling Rabastan Lestrange alone – his skin so flushed and red that you couldn't tell where his hair ended and face began. A split-second later, Dennis found himself joining Ron, and then his hands were so full his brain didn't have the time to think.

Yaxley went down first, and once they had him bound, stupefied and outside the house, it was now 4 on 1 against Rodolphus, who was apprehended soon after. The younger Lestrange, now up against impossible odds, succumbed seconds later, and they had just bound him and levitated him outside when suddenly, there was a loud explosion, and Dennis, already bruised and bleeding from his various battles, caught a chunk of brick to the skull and sternum and found himself slipping unconscious.

X-XXX-X

"What the fuck?!" groaned Ron, slowly getting to his feet, shaking plaster out of his shaggy ginger hair. He looked over at Harry already standing, grimly surveying the buzz of activity around them.

"Whuzzgoingon?" rasped Neville behind him, bleeding down both palms and knees. "I got you mate," said Ron, shooting a few quick Episkeys in his direction. "Thanks," the round-faced man responded, hauling himself upright with a grunt of pain before they both quickly headed back into the melee.

"Welcome back to the land of the living. Three major injuries – Dennis, Cornhill and Williamson; no casualties so far. I sent Maisy, Owen and the two Trainees to St. Mungo's with them – Maisy and Owen will be back soon though, to help with the movement of our two dozen strong prisoners. Rabastan Lestrange got pretty banged up but Terry patched him up – he should be okay for the cells, no need for St. Mungo's. Neville – can you and Terry oversee the transfer back to the holding cells? Send a Patronus for backup if you need – you can pull Leslie and Rose from St. Mungo's, or call some of the other Trainees working afternoons back at the Office. Ron – we need to go check out the house, mate." Harry jerked his head at the smouldering ruin behind them.

"Sure thing," said Neville, still looking a bit dazed. He kept eyeing the patch of ground where the two Lestranges lay, bound and stupefied, and his expression kept shifting between bewilderment, satisfaction, anger, and a little grief. "You okay, mate?" Ron asked, and it seemed to snap something into Neville, because he shook his head sharply, "yes, I'm fine."

Harry and Ron looked at each other a bit uncertainly, but now was not the time. They nodded at Neville, and then headed back where they'd come from, wands aloft, casting Aguamenti, strengthening charms, and Homenum Revelio as they went; stopping only to set up personal protective charms before entering.

X-XXX-X

"Shite, this basement is a dump," breathed Ron, as the two levitated themselves cautiously into the hole in the floor, every inch of their skin covered in protective charms.

The room was eerily silent, and pitch dark once the flames were extinguished. Everywhere they looked there was chaos – broken glass, shelves fallen to the floor, cauldrons melted and twisted with indeterminate Potions spilling in sickly pools along the floor and splattered across the wall. Harry suppressed the instinctive desire to gag at the burned eyeballs and spleens of indeterminate species littered at his feet. The smell was vile beyond imagination, despite the Bubble-Head charms they both sported.

"We need to get Savage here – this is a Potions lab. She's the only one who can help us identify what the hell is going on here, maybe figure out what happened. I'll send a Patronus."

Harry stepped forward, wand held aloft, light streaming through.

"Oh bollocks, shit, Ron, there's someone here!" he called out suddenly, recoiling back from the pieces of the blistered and burned body that was lying at his feet once he'd taken a step further. The urge to vomit grew – at the remnants of melted, livid skin fused to the scraps of clothing that had survived what must have been the epicentre of the blast, exposed, charred bone sticking out at unnatural angles, a ghastly cocktail of bodily fluids streaking the cracked and blackened floor.

"Fuck, in one piece?!" Ron responded, hurrying over as quickly as he dared. He stopped short when he saw what Harry had discovered, and his face turned a distinct shade of pale green.

"Not entirely," said Harry, dryly; and the two looked at each other and then down at the remains of the unidentified hostile with grim resignation.

X-XXX-X

When Dennis came to, he was at St. Mungo's, Leslie gripping his hand tightly, her body tense, only relaxing when he jerked back to consciousness. When she spoke, her voice was steady.

"How are you feeling?"

"Like a horde of stampeding Hippogriffs used me as a Bludger," he replied hoarsely, wincing at the sharp pain in his chest, seemingly a punishment from his body for his snark in the face of injury. "What…?" he trailed off as the effort required to speak proved too much for him, but she seemed to understand.

"You were the closest to the house, caught the worst of the blast - broke two ribs, punctured a lung, cracked your skull and ruptured a host of blood vessels, but they took care of most of that already. You should be out of here tomorrow. It's only been a couple of hours – they put you under when we brought you here. Williamson's going to have a wicked scar, but he'll live. Caught a Cutting Curse to the throat, they had some difficulty getting it to close. Healers say Maisy probably saved his life. Cornhill… she's going to be in the hospital some time yet. Some unknown Dark curse Lestrange threw at her – Owen saw it happen, said it looked like a purple flame. Cornhill got off a partial shield though, so Healers are hopeful, though she has a long road ahead…"

They looked at each other for a long moment. The pain in his chest was worsening, making it even harder to speak with every passing second.

"Explosion?" he managed, and once again Leslie provided him a succinct summary. The fear and worry in her eyes were the only signs of weakness she was allowing herself.

"We don't know what caused it. Rose and I have been in here the whole time – we got out of there as soon as the dust settled with you three, though she's gone back now to assist. Maisy and Owen went back to help out once they gave their reports to the Healers, but there's been no chance for a debrief. The others were taking the prisoners to holding cells and checking out the remnants of the property. The explosion came from below, so I wager there was a basement. We'll know more in a couple days. For now, get some rest, Den, it's time for your Pain Potion. I'm off work now, so I'll be here when you wake up. I love you."

X-XXX-X

A/N: Summary of the basement scene: Harry and Ron enter the basement to find a blown up and destroyed Potions lab, as well as a brutally disfigured body.

I have zero knowledge and background of how medical/criminal/forensic science works, so I yadda-yadda'ed some of that stuff there. Chalk it up to the differences between Wizarding and Muggle methods, please.

The Fidelius Charm bit was my absolute favourite. This was my attempt at reconciling some of the canon inconsistencies around the Charm.

Some main!case breadcrumbs may or may not have been scattered around in this chapter…

Reviews are like chicken soup for the author's soul :)