Beckett (Stana Katic) and Castle’s (Nathan Fillion) latest investigation of the murder of a man shot and left in an industrial cake mixer leads them to the shot caller of the Irish mob. While bringing in a bar owner named Siobhan (Cara Buono) with ties to the family for questioning the team accidentally blows Ryan’s (Seamus Dever) eight year-old undercover identity when the woman kisses him, and then slaps him, in front of his wife (Juliana Dever) and coworkers. A visit from Special Agent Sam Walker (Benito Martinez) gives them a motive for the killing when he reveals the victim was his confidential informant who was helping the FBI get their hand’s on the mob’s bible (a list of payoffs and transactions which could take down the whole crew).
With his Siobhan being hung out to dry by the FBI and likely to be killed by the infamous Bobby S. (Michael Rodrick) if she tries to run, Ryan volunteers to go back undercover and get the bible on his own. With no wire, transmitter, or back-up, Ryan resumes the life of Fenton O’Connell and jumps right back into the fire. Meanwhile back at the precinct, Beckett, Castle, and Esposito (Jon Huertas) work to try and tie Bobby S. to the murder, but the more they look into the case the more they become convinced someone in his crew is trying to frame their boss for a crime he couldn’t have committed.
Although Bobby S. welcomes Fenton back into the family, after a moment’s hesitation (when he considers shooting his old friend), neither Bobby’s new second-in-command (Brian Letscher) nor his wife (Christina Cox) are quite so willing to trust him. Even worse news for Ryan is the fact they’ve made Siobhan as a snitch and order Fenton to kill her to prove his loyalty. Thankfully, his partners are there in time to back him up and snag a whole boatload of bad guys.
It’s always interesting to see one of the show’s supporting players get to take center stage and “The Wild Rover” shows us a side to Detective Ryan we haven’t seen before, even if it does almost get him killed. The B-story mystery involving Beckett fixation on a name Castle whispered in his sleep is far less intriguing, but it does provide a nice moment between Beckett and Castle to close out the episode in which the writer gives her some back story into where his drive to become a great writer came from.