When subway passengers, including Detective Esposito, are taken hostage by a mysterious gunman, Castle, Beckett and the team must uncover the gunman's agenda before it's too late.

When an ER physician is gunned down in the street, Castle and Beckett soon discover that the victim was a mob doctor to a Mafia family run by an old friend of Castle's. Also, against the backdrop of Christmas, Esposito and Lanie reach a defining moment in their relationship.