When an ice cream vender is shot, evidence leads Beckett and Castle to believe that an unknown second-grader may have vital information about the case. Castle goes undercover at an elementary school to try and identify the second-grader but nothing goes as planned.

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.