Jason Beghe will guest star this fall on Castle.

On the show's third episode of the season, viewers will learn more about Beckett's early years on the police force. Beghe's character will play a key role in these revelations, as the actor will come aboard as Kate's training officer.

And we hate to break it to Castle, but there two were an item back in the day.

Beghe most recently appeared on episodes of Hawthorne and Californication.

Castle returns on September 20.

Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.