Checkmate is the fourteenth episode of the fourth season. It was released on 11 April 2008.
A.U.S.A. Robin Brooks is back in town, and her involvement in Don's case stirs up memories and feelings as they try to keep witnesses alive. Meanwhile, David attempts to bond with a young man who's in the chief suspect's thrall.
The chess term for the 'killing stroke', its imagery is somewhat reflected in the next episode, although logically the endgame leads to checkmate not vice versa.