Dr. John Watson resumes his medical career as the head of a clinic dedicated to treating rare disorders six months after the passing of his friend and partner, Sherlock Holmes, at the hands of Moriarty.

Watson and the fellows investigate the perplexing case of a family man with a bullet lodged in his head who insists he is a Scottish soldier and seems to be hiding something nefarious. Meanwhile, Watson looks for clues to whom his estranged wife Mary is dating, and Ingrid tries to work out what her "tell" is when Watson calls her out for lying.

Watson and the fellows treat a comedian who collapses at an open mic night and seeks their help, but throughout the investigation her past is revealed to be much more complex than initially believed. Meanwhile, the team becomes concerned when Watson himself collapses and suffers another traumatic brain injury as he continues to hide his worsening symptoms.