Murder investigation launches Castle into what seems to be an alternate universe where he's never met Kate Beckett. Castle has to convince Beckett, Esposito and Ryan -- now complete strangers -- to let him participate in their investigation so he can find his way back to his own world. This inspires Castle & Beckett to finally take a trip to the altar.

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.