Jim and Sherlock aren't really human. Well, they are, but only because they chose to become that way. Really, they're the latest forms of the Doctor and the Master (and yes, I know that that gets in the way of the most recent Dr. Who show, but you can reason it out through parallel universes or just disregarding certain aspects of Dr. Who). After the Time War, each thought he was the last of his kind and used his fob watch to become human. Despite this, the two were still drawn to each other and their human versions formed a rivalry.

Each were fairly happy with their lives, until one (and it's up to filler which one)found a watch in his bureau and opened it. Suddenly, overwhelmed with all the memories and knowledge that the Time Lords are almost extinct, he couldn't help but want his childhood friend back and sets about getting the other to open his watch.