From SheWhoScrawls:

December 28: The Baker Street Irregulars encounter a yeti while out snooping for Holmes.

They had searched high and low for the man who had been missing for three days. He had told Watson that he'd be back on the fifteenth, and it was now the eighteenth. It had been Wiggins' suggestion to check one of the underground rail lines that was under construction. It was an unexpected place to hide, therefore it would be perfect if one were being persued.

As they wandered the dark tunnels after hours, pushing the lantern that Doctor Watson had so thoughtfully lent them into each dark corner, half afraid that they would find a grievously wounded Sherlock Holmes lurkong in front of them, little Peterson ran into something large and furry. Peterson's scream of shock and terror had been everyone's cue to run.

Eventually, curiosity and sense of duty won out over fear and the boys gathered and returned to the spot where Peterson had run into the massive brown creature, half-afraid that it had gotten Holmes and that it would get them too. Rather than having gone off in search of them as they thought it would, the creature was standing where it had been left, completely stock still as if it were a taxidermy animal. The creature's eyes were dark and dead and reflective as glass.

"Someone must've left it down here as a prank." Wiggins decided after the creature continued to not move.

Laughing in relief, the boys began to feel foolish and needled the others for running, commenting that they were braver than whichever boy they chose to point out and that they'd only left because everyone else had done so and that they hadn't wanted to be left without the lantern. Before a brawl could break out, a wind blew through the tunnel which had previously been dead calm and a loud wheezing grinding noise could be heard.

Deciding that discretion was the better part of valor, the boys rounded the nearest corner which was in the opposite direction of what they had decided was the source of the sound which was difficult to pinpoint in the echoing tunnel. Eventually, the noise stopped, and the bravest of the boys peered around the corner trying to see what had appeared in the dark. Without warning, a door opened and a bright shaft of light pierced the darkness, nearly blinding the boys who had grown used to the dim light of the smoky lantern. Two men stepped through the doorway, one a man with close-cropped hair who was wearing a strange looking leather jacket, and the other Sherlock Holmes.

"Here you are, only three hours late." the man with the short black hair who spoke with a Northern accent said.

"Thank you." Holmes replied, before walking off and nearly running into the large and furry creature that Peterson had run into.

"What the devil is this?!" Holmes exclaimed.

The man in the leather jacket came running, practically skidding to a stop before the creature. After looking stunned for a few seconds, he began to examine it alongside Holmes. Based on how he was doing so, it was apparent that he knew what the creature was, whereas Holmes did not.

"You're early, very early." the man said after his examination, sounding curious. "Though, I wonder what deactivated you..."