Castle and Beckett get engaged. She moves to DC for her new job and Castle sets out on a book tour. After not seeing each other for six weeks, Castle heads to DC to see his fiancé. He gets involved in her case and is abducted by a criminal who has been poisoned with a toxin. Turns out the toxin is airborne and it is in Castle’s blood. He’s got less than a day to live.
Alexis returns from Costa Rica with a new boyfriend. Ryan and Espo are still carrying on their bromance. Ryan is prepping for his new baby.