Best selling mystery writer Rick Castle has it all. A mother who makes him crazy, a teenaged daughter he adores, and successful career...until he kills off the main character in his latest series of novels. Writer's block (and his ex-wife / publisher) plague him until Detective Kate Beckett hauls him in for questioning when someone begins committing the murders from his books in real life. 

As Castle helps find the real killer, he becomes hooked on solving real life crimes, especially with Detective Beckett. After pulling strings with his friend, the mayor of New York City, he begins shadowing Detective Beckett on all of her cases.

Much to Beckett's dismay, Castle finds inspiration in her for a new character, Detective Nikki Heat and a new series of mystery novels. 

The two begin to become friends as they solve unique homicides that include a body stuffed in a safe, a frozen corpse, and even find a missing child. But when Kate Beckett tells Castle the tragic story of her own mother's murder and how it's gone unsolved for a decade, she asks one thing. That he leave the case along. When Castle tries to help bring her closure, will it be the final straw in their tentative partnership?