"A lizard? Sherlock, really. A lizard?" John grumbled as he threw down the bags from the pet shop and flopped into his usual chair. Sherlock, gingerly carrying a heavy cardboard box containing a rather surly male Western Green Lizard, slammed the front door shut with his foot as he made his way toward the living room. An enormous tall aquarium (which Sherlock had procured earlier in the week, without informing his flatmate what he had in mind for its use) was already settled in the corner, its built-in waterfall burbling merrily. Sherlock had set out crickets in the morning before dragging John to the pet store, and they wandered around the moss and rocks with dazed looks on their tiny insect faces.

"Oh, please, as if you've never brought home anything I found repugnant or annoying."

"If you're talking about Sarah-"

"Of course not," Sherlock replied airily, settling down on the couch and putting the box beside him. "I want to wait a bit before we put him in his new home. It'll calm him down a bit to give him a transitional period in the quiet of the box before he's thrust into yet another alien environment."

"Why exactly have you got a lizard in the first damn place?" John punched the Union Jack pillow irritably as he re-settled it behind his back. Winter always made his shoulder ache ever since he got back from Afghanistan, and carrying the enormous plastic bags filled with lizard ephemera – including vitamin drops, a basking lamp, a humidifier and thermometer, and what he could only assume was the damn beast's auxiliary food beyond the doomed crickets currently prancing in the splashing waterfall – had agitated the sore muscle.

Sherlock noted the pain flashing across his face and stood up to retrieve his flatmate's pain cream from its place on the mantle, beside the jack-knife impaling the mail. "Because I wanted one. It's my late Christmas present to myself."

"Right, along with the new microscope, the expensive glass slides, the dissection kit, and the 17th century violin with a retooled horsehair bow. You really know how to treat yourself."

"It's been a difficult year, John. There's no harm in buying myself a few treats. And I bought you a nice new coat and a book about forensic pathology, so you have no right to complain." He sniffed, as if offended, as he thrust the pain cream into John's hand.

"Right, but you can barely take care of yourself. Reptiles are notoriously difficult pets – what makes you think you will honestly be able to keep this thing alive longer than a week?" John took a glance at the box, which was vibrating as the lizard threw himself against the walls of his tiny prison. He imagined that if it were capable of speech, a long string of explicatives would issue loudly from the box until Sherlock finally picked up him and placed him in his new home.

"He has many the same qualities as myself, John. Rarely requires food or companionship, prefers to be left to his own devices, inhibited expression of emotion. We'll get along famously. And it'll nice to have someone about who appreciates my … reduced humanity." Sherlock finally went to the box and plucked out the lizard, who latched onto him as if they'd known each other forever. John could have sworn that Sherlock actually nuzzled the creature before dropping him into the readied aquarium and clasping the top shut.

"Right. And what are you going to name him?"

Sherlock smiled. "Gladstone.