Captain Gates (Penny Johnson) calls in Castle (Nathan Fillion) with an unusual situation. A woman (Alicia Lagano) fleeing from the murder scene of her boyfriend has barricaded herself in a dentist’s office with five hostages and the only one she will speak to is the mystery writer. After meeting her face-to-face, Castle discovers the gun-wielding crazy lady has only a single demand – that Richard Castle solve the murder of her boyfriend and prove her innocence. With her fiance occupied talking with his number one fan, the still unemployed Beckett (Stana Katic) convinces Gates to let her work the case.
Despite all the evidence to the contrary, Castle believes the mentally-unstable woman off her medication may have actually been framed for the crime. Even after learning his fan has a troubled past involving stabbing her foster brother as a juvenile (in a situation eerily similar to her boyfriend’s death) and getting shot in the chest when his fan struggles with one of the hostages, Castle still believes her innocence and even thinks he knows why she was framed for murder.
Beckett and Ryan continue to investigate the case and track down a tight-lipped lawyer who paid a file clerk (Charlie Bodin) to illegally unseal the woman’s juvenile file. Even if Ryan (Seamus Dever) can’t get the lawyer (Patrick Heusinger) to roll over on his client, Esposito (Jon Huertas) and Sullivan (Joshua Bitton) strike gold by discovering the true parentage of Castle’s fan – a powerful gubernatorial candidate (Tom Amandes) who has never claimed his illegitimate daughter. Although the politician turns out not to be their murderer, his connection to the case helps catch the real killer.
Even if she isn’t working in any official capacity, “Number One Fan” allows Beckett to take control of the investigation (and earn some good-natured ribbing from her colleagues for trying to order them around) and help prove Castle’s hunch. The nature of the episode also puts Castle in the middle of the action in a different manner than usual. By the time the closing credits roll Beckett’s professional future will be back in track and Castle will have saved the life of the woman who shot him (and took five people hostage) but who also was innocent of the crime she was initially charged with.