Martin hits his head and starts to think he's Sherlock Holmes. It's up to the author whether Sherlock is a real detective who lives in London or fictional character from ACD's books and BBC TV Show, but Martin definitely isn't him, doesn't have a mystical bond with him, wasn't him in the past life etc. Martin's deductions are crap, his attempts at "Sherlock voice" are a source of amusement to everyone, the police arrest him when he tries to trespass on their crime scenes, he sees nonexistent crime/plots everywhere etc.

tl;dr Martin thinks he's Sherlock, but utterly fails at doing Sherlock-y things.