Fillion plays mystery novelist Richard Castle, who is consulted by Detective Kate Beckett (Katic) of the NYPD after a killer begins copying murder scenes from Castle's books. When an obsessive fan is caught too quickly, it bothers his writer's sensibilities; the capture was too easy. Castle realizes that the man the police have arrested was framed, and helps to capture the real murderer. Enamored with the idea of solving crimes -- and a bit more enamored of working with his beautiful "muse" -- Castle calls in some favors and gets permission to do some research for his new series of books, starring a character based on Beckett. Together, They Fight Crime! Castle and Beckett have some excellent on-screen chemistry going.
An interview with Fillion providing a lot of relevant information can be found here.
- As of "Always", it is finally resolved. Sort of..