AN: Wow, another update! I'm on a roll!

Okay, this one's pretty long but I couldn't find a place to stop it.

Incidentally, I shouldn't think anyone's under any delusions on the topic, but if I owned Sherlock I'd be running around London in a swishy coat solving crimes. So.

Also, I have researched this chapter, but unfortunatly I have absolutely no clue how IPD's are manufactured and tested, so please take that as 'magical plot hole powers' of being right even though it isnt :)

III

Sherlock and John were at that moment occupying themselves as they frequently did, ie sprinting through the streets of London hot on a trail. Or at least, Sherlock was. John was struggling to keep up and cursing inventively the day he chose to move in with the sociopathic Consulting Detective.

'Taxi!' Sherlock hollered, flagging down a passing cab. John shuddered slightly as he got in. He had had a mild phobia of taxi cabs ever since their first case.

The cab pulled away from the kerb and swung out into the seething London traffic.

'Where to?' enquired the cabbie.

'Liverpool Street station,' Sherlock responded idly. The cab set off.

'Sherlock,' John said when he had got his breath back (he was really far less fit than he should be, as a retired Army medic and a flatmate of Sherlock he should be in the peak of physical fitness) 'Where exactly are we going?'

'Oh come on John, surely it's obvious.'

'The only thing that feels obvious right now is that I'm never going to get to finish that coffee.'

Sherlock sighed. 'Come on, John! Why would these men, who we can assume had no death wish and were not coerced, walk into a patch of deadly gas?'

'Well, I can't…'

'Use your imagination. Think hypothetically. The only reason for someone to walk into a death trap is?'

'That they had no other choice.'

'Or?'

'They didn't…' John felt frustrated. He hated it when Sherlock made him do his thinking for him. Especially when Sherlock already knew the answer and could have simply told him. Suddenly, light dawned. 'Unless they didn't know it was there!'

Sherlock clapped his hands together. 'Well done, John. Precisely. They did not know that there was any danger.'

John felt a headache coming on. 'How is that possible? The gas detectors were perfectly functional.'

'Yes. And here you come to my second point. How do we know that they were functional?'

'They were tested.'

'By whom, John?' Sherlock said impatiently.

'By the… oh.' John stopped.

'Precisely. The only people with the power to have tampered with them in the first place. The manufacturers! And Lestrade sent them straight back where, I have no doubt, the perpetrators immediately fixed them to hide all evidence of their crimes.'

They sat in silence for a moment.

'So where are we going?'

Sherlock sighed. 'Sometimes, John, I despair of you. We are going to see the manufacturers, of course. The firm is called Erik and Moore gas detectors. Erik and Moore are presumably the senior partners and therefore under most suspicion. The firm is based in Essex, so we are going to catch the,' he consulted his watch, '3.45 train to Ipswich from London Liverpool Street. Questions?'


An hour later, Sherlock and John were standing outside a grubby workshop on an industrial estate. 'Senior Partners', it turned out, was far more glorified than this dead-end dump intimated. Sherlock glanced up at the sign- 'Erik and Moore gas detection Ltd' and pushed open the door gingerly. It sagged slightly.

A row of strip lights illuminated the gloom, revealing a bare concrete floor and mould. There was a desk propped up in one corner. Against all probability, it had wads of paper under all four legs.

Half the room, however, was taken up with a substantially more impressive sight. It was a glass chamber, completely sealed, with big red warning notices pasted over the side and a row of bio-suits hanging by the door. Inside, it was lit brightly by a brand-new light. A red LED over the door burned brightly, and a large warning notice proclaimed that the chamber was currently unsafe to enter. Inside stood a man in a sealed bio-hazard suit, holding a small cuboid which was flashing slightly. The man looked up, saw John and Sherlock and visibly jumped. He raised a hand- in greeting or warning, they couldn't tell- and made his way towards the door which, John saw, was designed like an airlock. There was a slight hiss, the light became green, and the door swung open. The man clumped out and without a word to John and Sherlock dumped the device on a workbench. He unzipped the bio-hazard suit and hung it on a peg.

'So, what do you want?' he asked gruffly. He had an Essex accent and a potbelly. He hardly looked like a master criminal. Then again, John reflected, nobody did. Aside from Mycroft. Who wasn't, so it didn't count. Unless being the British Government made you a criminal- which it probably should if the amount of tax he'd been paying was anything to go by.

John shook his head and returned to the present.

'I would like to meet Mr Erik and Mr Moore, the owners. Is that possible?' Sherlock was asking smoothly.

The man laughed. 'Yer looking at Mr Erik. Call me Will, s'friendlier. Kyle- Mr Moore- is around the back.' He raised his voice. 'OI! KYLE! Get yer ass in here, we got visitors!'

Footsteps sounded around the edge of the huge chamber. A dapper looking man emerged from around the corner. His dark hair was spiked slightly and he wore a ACDC T-Shirt. His converse shoes were battered but neatly cleaned. He was poor, but took pride in what he had. John shuddered. He was beginning to think like Sherlock.

'What can we do for you?' asked Moore. For some reason, John took a more or less instant dislike to him.

Sherlock stepped forwards. 'We're here to enquire about testing methods for gas detectors. There's been a spate of failures recently, as I'm sure you're aware, and we want to reassure people that there is no lapse in security or safety. Unfortunately, we aren't so familiar with the process and we'd hate to get it wrong.' He smiled tightly.

'Sure, no problem mate,' Erik responded, moving to pass them two bio-hazard suits. His partner held him back.

'Hang on a moment, Will.' His dark eyes inspected John and Sherlock. 'I'm sure you understand, gentlemen, but I'm going to need some identification from you two. Who do you work for?'

John began to panic slightly, but Sherlock simply reached into his coat and removed a laminated ID badge. 'I'm Simon Harris, I work for the PR arm of the United Kingdom Mining Union. Our members have been worried, obviously, so I'm just here to give them some reassurance. This is John Smith, he's my junior. Neither of us are hugely familiar with the process, so…'

Moore still looked suspicious, both of them and just generally suspicious (he had really shifty eyes) but he nodded and let his partner move on.

'So basically,' Erik told them enthusiastically, 'This is a giant gas chamber. If you were to be stuck in here without a suit, it could get really nasty. This is top-of-the range stuff, we have to conform to really strict guidelines here. You just pull on an airtight suit and stand in there with the detector you're testing. We can pump in variable amounts of all four dangerous gases we test for; whitedamp, blackdamp, firedamp and stinkdamp.'

Sherlock nodded like he knew what that meant. John was willing to bet he did, too.

'Kyle, can you run the gas for me?' Erik called. Moore nodded and went back behind the tank. Erik pulled on his suit again. 'Right, if you want to go around the back, Kyle will show you how the controls work. You need to operate the gas input from outside.' Erik keyed in a code with clumsy gloved fingers and the outer door slid open. He picked up the detector and entered the airlock. Behind him, the door slid closed and the light turned red.

'John Smith?' muttered John as they made their way around.

'Well, I didn't want to overload you too much. You might not have been able to remember a different first name and I don't want you giving us away.'

John rolled his eyes.

'Isn't that the most obviously fake name you could have picked?'

'It would be,' agreed Sherlock, 'If it wasn't for the fact that everyone knows that John Smith is an obvious fake name, and therefore wouldn't pick it as an alias, so therefore the only people using it are not using a fake name.'

'That was convoluted.'

'Welcome to the world of crime,' Sherlock muttered.

They rounded the corner to the other side of the chamber. Moore was standing at a bank of complex looking controls.

'Do you want an explanation of these?' he asked, looking up. He didn't look altogether friendly.

'If that's okay,' Sherlock responded.

'Right. These four here,' he indicated four large buttons, 'control which gas we pump in. These sliders control the amount. The first test I'm going to do is with Carbon Monoxide.' He slid the sliders down to the bottom again, then hit the big button labelled CO. Inside the chamber, a sign lit up with 'Carbon Monoxide' written on it. Erik gave a thumbs up from inside his suit and flicked a button on his detector.

'As I slide up the slider,' Moore continued, 'the quantity increases. Like so.' He raised it up. A readout in front of him showed the gas level inside the chamber. At present, it was a normal mix of Oxygen, Argon, Hydrogen, Helium, and Carbon Dioxide. Levels of CO were low but as Moore manipulated the controls it began to rise. The bar flashed from green to red at the exact moment that the device Erik held began to beep. Erik gave a thumbs up to Moore, who nodded and slid the slider back down.

'We don't actually empty out between tests,' he explained, 'but we do purge the chamber regularly. We clear it to a safe level of each gas before we begin the next one, so as to not contaminate the readings.'

Sherlock nodded. A light on the console blinked green.

'Ah, there we go. Next gas,' Moore commented. He pushed the button labelled CH4. The sign in the chamber winked out and was replaced by one reading 'METHANE'.

As before, the device beeped just as the bar turned from green to red.

'That colour change means Dangerous,' Moore explained. 'It's actually still a safe level, but it's high enough to be potentially dangerous. We have the boundaries so low because that gives people a chance to get to safety.'

John & Sherlock both nodded. John felt like this was the first thing he'd understood all day.

The testing continued. When all four gases (Carbon Monoxide, Carbon Dioxide, Methane and Nitrogen, as well as compounds of the four) had been tested, Erik made his way to the airlock and slammed his palm against a release button. Sherlock, John and Moore walked around to the other side of the tank.

'So, Mr Erik, what position do you hold in the company?' Sherlock enquired. He had produced a notebook, presumably for veracity's sake, and was taking notes.

'Well, Mr- Harris, wasn't it?' Erik checked. 'Right, Harris. Okay, well technically I'm a senior partner and co-CEO. Not that that means much! S'just me and Kyle here, so really I'm the program guy. I do all the software stuff on these. Kyle's the one who actually assembles them, although we do each others jobs sometimes if one of us is off sick or summin.'

'Alright, thank you, Mr Erik. And what would you say your position in the company was, Mr Moore?'

'Like he said, I'm a Senior Partner and all but I do the hardware. Works out quite nicely, really.'

'Okay, thank you,' Sherlock said, pocketing his notebook. 'I think that should be it. If we need anything else, we'll phone up, is that OK?'

'Sure, any time. D'you want anything before you go- tea, coffee?'

Sherlock shook his head. 'Sorry, we have to catch a train. Come on, John.' He turned and swept out. John gave an apologetic smile and shook hands with the two men before hurrying after him.

AN: Incidentally, I'm from Essex. That's why I based it here. Also, trains from London Liverpool Street to my nearest station are 45 mins; I made it an hour to account for the fact that they are going further down the line and also travel time ect.

And if anyone wants to correct me on IPD testing methods, feel free. I could use the info.