Castle and Beckett may be not experts at love, but these two have always been proficient at one aspect of their lives:

Catching criminals.

That will change on season three, however. Castle creator Andrew Marlowe tells TV Guide that the show will introduce an "incredibly manipulative villain" in the fall, one known as "3xK" because he kills in three.

"[He] will get away and become a thorn in their sides," Marlowe said, adding this killer "will have the delicious appeal of a Hannibal Lecter."

Castle returns with new episodes on September 20.

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