Castle and Beckett pursue a ruthless killer who, they discover, is part of a sinister, international conspiracy. Castle and Beckett are joined on the case by Sophia Turner, a smart, sexy CIA operative who has a complicated past with Castle.
Lily and Marshall invite Robin and Kevin on a Valentine’s Day trip to Vermont. Kevin and Robin use the trip to talk about taking their relationship to the next level. Meanwhile, Barney realizes he has finally met his match.
Marty goes to war with his son’s school when Roscoe is accused of sexual harassment for trying to kiss a boy. On the home front, Marty’s personal life is further complicated when April, a stripper he met in New York, suddenly reappears. Meanwhile, Jeannie starts to get cold feet at the prospect of her upcoming wedding.
Peter, Olivia and Walter come face to face with a mysterious and terrifying Fringe event as they get trapped in a town that there’s no escaping from.
While a jury deliberates the guilt or innocence of a defendant on trial for murder, Jane tries to prove she has been wrongfully accused before time runs out. Meanwhile, Summer may have gone too far when she helps Cho make a case against a suspect.
Phil’s preoccupation with a business opportunity blinds him to his new partner’s inappropriate behavior around Claire. Jay and Gloria may regret letting Mitchell and Cameron be their temporary house guests when their home is being fumigated.
Frankie and Mike decide to try a fresh approach to Valentine’s Day by having dinner out with friends. But a clueless Mike angers Frankie by refusing to present her with a simple romantic gesture. Meanwhile, Axl can’t think of a life-changing moment that’s occurred in his life when he’s assigned to write a school paper on the subject, Brick is tasked with writing a report on love, and a hapless Sue doesn’t understand why her new boyfriend has suddenly turned into a bad kisser.
Neal gets a taste of what his ideal life in New York might be like when he is assigned to work a case with his ex-girlfriend, Sara Ellis.
When Castle and Beckett investigate the killing of a treasure hunter, they discover the case is linked to a mysterious homicide from 1947 involving a hard-boiled private detective. Castle realizes the only way to solve the present-day murder is to solve the murder from the past. The 1947 case comes to life through stylized flashbacks, featuring Castle as the private eye and Beckett as a femme fatale.
Lily asks for her father’s help when she and Marshall decide to throw a housewarming party. Marshall’s boss gets under Ted’s skin and the two nearly come to blows, and Barney puts the moves on a wacky divorcée.