For the vegetation that gives Mars its red appearance had taken root on Earth. As Man had succumbed to the Martians, so our land now succumbed to the Red Weed.

THE UNDERGROUND TUNNELS

As soon as they'd all descended down the ladder Sherlock handed them all torches that seemed to magically appear out of nowhere. Then without a word he set off, striding down the tunnels with the others trailing after him.

After about twenty minutes John couldn't contain himself any longer. "Sherlock."

"Hmmm," came the non-committal response from up ahead.

"Care to share exactly where it is that we're headed?"

Sherlock came to an abrupt halt before turning back to the others, his furrowed brow clearly indicating his annoyance. "Was I not clear?"

Seeing the blank expressions of his companions, he let out an exasperated sigh. Reaching into one of the pockets of his Belstaff he withdrew a map of the tunnels that he'd hastily drawn up and handed it to Lestrade. "You'll need this to make your way back."

"So we're splitting up then?" Lestrade clarified.

"Yes. Molly and I need to get to Barts to collect some equipment. I need you, John and Mary to gather as many samples as you can."

"Samples?" John queried.

"Martian DNA," he paused briefly. "I know what I'm asking of you is extremely dangerous. But to defeat this enemy we have to know exactly what we're dealing with first. As far as I'm aware no such undertaking has ever been done, unless Area 51 has something. But even if communications weren't down, they're unlikely to share their findings if they had any. So we're going to have to get some ourselves."

"Fair enough," John responded, " Once we have the samples, then what?"

"Then we look for weaknesses. These Martians clearly have some or they wouldn't be using human blood in an attempt to adapt to our atmosphere. Once we can pin down the exact nature of their weakness then we can exploit it, and use it to our advantage."

"Sherlock," Mary called out as the detective once again took off down the tunnel.

"What now?" he demanded turning back.

"What are we supposed to put these samples in?"

"Oh." Sherlock paused in thought, then began frantically searching his other pockets, eventually producing a number of small sample bottles, flick knives, some labels and markers which he handed to Mary, John and Lestrade.

"Do you have an extension spell on your coat pockets like in Mary Poppins and Harry Potter?" Lestrade asked in amazement, he and the others curious what else the detective kept stored away in his coat.

The references were completely lost on Sherlock, who looked at them all as if they'd lost their minds.

"I have a long coat with many pockets," he said, stating the obvious and frowned with confusion when the others laughed affectionately at him.

John patted him on the shoulder. "Mate if you don't want to be a rubbish godfather you'd better set up a room in that Mind Palace of yours dedicated to children's literature."

Sherlock rolled his eyes before turning and continuing down the tunnel. Over his shoulder he stated. "I'll have you know I'm a brilliant godfather and will share with Isabelle when she's old enough my knowledge of Grimm's Fairytales. The actual fairytales and not the saccharin Disney versions."

Ten minutes later Sherlock paused near an exit. "This will take you close to New Scotland Yard," he said. "Get as many samples as you can."

"And take care," Molly added before hurrying off after Sherlock who was already several feet ahead.

OUTSIDE ST BART'S HOSPITAL

The building still stood but it had sustained significant damage, having clearly come under attack from the invading Martians.

Sherlock led them quickly inside to the hospital foyer. Molly paused to observe the damage, chewing nervously on her bottom lip as she looked up at the ceiling.

Deducing her concerns Sherlock attempted to reassure her. "We should be fairly safe as long as we don't do anything that could further compromise the integrity of the support structure."

She nodded her head absently before following the detective who was heading for the nearest stairwell.

As soon as they reached the familiar surrounds of the pathology lab, Sherlock made his way to the staff locker room.

It didn't take him long to locate what he was looking for. In one of the storage cabinets he found a number of large bags that would be big enough to carry the items they were there to acquire.

They made quick work of gathering up microscopes, Bunsen burners, glass beakers of various shapes and sizes, thermometers and a wide range of chemicals that they hoped could prove useful to their investigation into how to best tackle this new and extremely destructive adversary.

As they made their way back to the manhole, Molly turned back once more to look at the senseless act of wanton death and destruction. "Why did they do it, Sherlock?"

Whatever he intended to say died before he could voice it when he saw the way she looked at him. Instead he took the time getting them both back safely in the tunnel to consider his reply. When he did he responded carefully. "I don't believe they had a specific reason, at least not at first. Their initial motivation appears to simply be one of conquest. But when they realised they were having difficulty adapting to our atmosphere, they needed to find a quick and easy means to absorb what they needed to survive here."

"So they believe that injecting our blood directly into their veins will give them some sort of protection, an immunity boost to the Earth's atmosphere?"

"Precisely," Sherlock replied, giving her a rare smile that faded all too quickly. "Unfortunately to get enough blood into them, they require a lot of…"

"Fresh supplies," Molly finished.

"Yes," he confirmed, adding quietly. "And the hospital would be the perfect place to obtain ready provisions…"

Molly shuddered, imagining the helpless and terrified patients, assuming they survived the initial attack on the hospital, trapped and unable to escape, picked off one by one.

Sherlock placed his hands gently on her shoulders. "What's done is done Molly," he said as he pulled her in close for a reassuring hug, before swinging the bags over his shoulder and taking her hand in his and leading her through the tunnels once more. "We need to get back so we can start on our solution to this terrible catastrophe."

NEAR NEW SCOTLAND YARD

When John, Mary and Lestrade emerged from the underground tunnel, they found themselves looking upon the landscape of another planet.

The London that they all knew and loved no longer existed. Mars' red weed had taken root on Earth.

Wherever they looked the red weed clung, strangling all that it touched. Its ability to adapt was clear to see, as it grew with frightening abandon. Its claw-like fronds spreading out in all directions, overpowering and consuming everything it came into contact with.

Without a word John bent down and collected a sample.

They moved cautiously down what had once been familiar streets. Streets that were now broken memorials to what had once been. So many buildings stood in ruins. Those that weren't had still suffered damage. Looters had been busy, grabbing what they could. Shops with their windows broken, the contents from within scattered all over the pavement.

Bodies also littered the streets, their outlines softened by a layer of black dust, a residue from the poisonous smoke that filled the air. This was another weapon developed by the Martian's to subdue the local population.

Mary carefully scraped a quantity of the dust into another sample bottle.

Lestrade, who had gone on ahead suddenly paused next to a pile of bodies that appeared to have been unceremoniously dumped in the street.

"John," he called out as loud as he dared. "Come look at this."

John and Mary quickly made their way over to the detective inspector.

"What have you found?" Mary asked.

"All the other bodies around here, they've clearly been killed by the heat ray or from being in or near a building that's been destroyed, yeah?" Lestrade said. "But look at these bodies, they're all in pretty good nick, except…"

"They look as if they've been drained," Mary finished excitedly.

"Look for where the Martian's made their incisions," John ordered as all three began searching the bodies. "There may be some DNA transfer in the wound."

They had just finished getting the samples they needed when out of the gloom a fighting machine materialised. With nowhere to hide, and not wanting to draw attention to themselves their only option was to remain perfectly still.

The enormous tripod figure looked from right to left, and then back again before moving on.

"That was close," Mary noted once they were certain they were safe.

"Time to head back then," Lestrade remarked hopefully. "We've got enough samples."

"We've got all we can" John agreed. "Let's just hope it's enough to give us the answers we need."

THE DIOGENE'S CLUB BUNKER

As soon as Sherlock and Molly had returned they began setting up the equipment they'd retrieved from Bart's.

By the time John, Mary and Lestrade joined them they were ready to begin work on the collected samples.

Mary relieved Mrs Hudson of little Isabelle. The elderly landlady took the opportunity to go have a well-earned nap, with the aid of some herbal soothers, while Lestrade and John took over Wiggins watch over Mycroft.

Sherlock and Molly worked quietly and efficiently together. Each anticipating the others needs like a well-oiled machine. They sat side by side, both focussed on the samples they were analysing under their respective microscopes.

It soon became clear that all the samples had something in common, they all showed a varying degree of bacterial infection.

Sherlock sat back in his chair to consider what his eyes were showing him. "Could it really be that simple?" he mused aloud.

It was clear that no sooner had the Martian's arrived on Earth than these microscopic predators and pathogens had set to work attaching themselves to, while feeding on and distorting the alien DNA makeup with the infections they carried, infections, completely unknown on Mars.

As these microscopic allies multiplied, they spread quickly through this new ingestible food source, gradually destroying the invaders from within.

The implications were so much more than they could ever have hoped for.

Sherlock watched as the same realisation came to Molly. Turning to Sherlock she asked. "So what's our next move?"

"We need to speed up the process," he replied. "We have to act quickly before the Martians adapt and become immune to their affects."

Molly sat in thought for a moment. "Well heat is the simplest agent we have at our disposal to increase the growth of most bacteria," she noted.

Sherlock stood up. "Brilliant," he replied, then to Molly's surprise he bent down, took her face in his hands and kissed her firmly on the lips. "Then that is where we will begin our experiment."

Molly sat in stunned silence as she watched him make his way over to the burners. 'Had he really just…'

"Come on Molly, we've no time to lose."

Still flustered Molly somehow managed to get up and made her way over to where he was already getting things ready.

A couple of hours later they looked at the small sample they had created.

"It's not going to be enough to destroy them all." Molly noted.

"No," Sherlock agreed. "But it will hopefully be enough to test our theory."

"And how do you propose to do that?"

Sherlock looked down at her, an enormous grin spreading across his face. The game was clearly on. "By getting as close to them as I can," came his foreboding response.