In the Thin Man series, who was the "thin man"?

Nick Charles (Powell), a retired detective, and his wife Nora (Loy) are attempting to settle down. They are based in San Francisco but decide to spend the Christmas holidays in New York. There he is pressed back into service by a young woman whose father, an old friend of Nick's, has disappeared after a murder. The friend, Clyde Wynant (Ellis) (the title character "thin man"), has mysteriously vanished. When his former secretary and love interest, Julia Wolf, is found dead, evidence points to Wynant as the prime suspect, but his daughter Dorothy (O'Sullivan) refuses to believe that her father is guilty. She convinces Nick to take the case, much to the amusement of his socialite wife. The detective begins to uncover clues and eventually solves the mystery of the disappearance through a series of investigative steps.

The murderer is finally revealed in a classic dinner-party scene that features all of the suspects. A skeletonized body, found during the investigation, had been assumed to be that of a "fat man" because it is wearing oversize clothing. The clothes are revealed to be planted, and the identity of the body is accurately determined by an old war wound to the leg. It turns out that the body belongs to a "thin man" — the missing Wynant. The double murder has been disguised in such a way as to make it seem that Wynant is the killer and still alive. The real killer is uncovered at the dinner party, before he almost takes the life of someone who knows too much.