Talk about the stakes being raised... In the conclusion to a two-part story (begun in "Pandora"), Castle and Beckett race to find the "Linchpin" (whatever that is) that will stop a chain of events leading the United States into war. Jennifer Beals guest stars as CIA Agent Sophia Conrad (the inspiration for Castle's Clara Strike character in the "Derrick Storm" novels) and Josh Stamberg guest stars as intelligence officer Martin Danberg.

"Linchpin" will air on Monday, February 20 at 10pm on ABC.