Dr. John H. Watson, a retired combat medic whose health was compromised during service in the First Anglo-Afghan War, returned to London seeking accommodation. On the suggestion of his friend Stamford, Watson visited 221B Baker Street to meet potential flatmate Sherlock Holmes. He discovered Holmes lying in a pool of blood resulting from an experiment with a blood-reactive chemical agent. After confirming Holmes was unharmed, Watson found his own background as a war medic and writer swiftly deduced by Holmes through observation. With landlady Ms. Hudson's approval, Watson moved in.

Watson possesses strong deductive skills honed through frontline medical experience and adheres to a strict moral code. This is evident during their first case when he drew his service revolver on Holmes to prevent him from shooting a suspect, Jefferson, who had offered information in exchange for a mercy killing. Holmes revealed he never intended to fire, planning instead to take Jefferson to the police. Watson chronicles Holmes' investigations for the Strand Magazine under the pseudonym "Conan Doyle," fictionalizing elements for narrative purposes while documenting their collaborative work.

His military background informs his practical nature and composure during investigations. Watson actively participates in crime scene analysis beyond providing medical expertise, offering grounded perspectives that sometimes aid Holmes' deductions. Their dynamic evolves from initial flatmates to trusted partners, with Watson serving as both collaborator and biographer.