The killing of a police rookie with an NYPD academy service gun is tied to the present class's female top student, so Becket, the best ever, goes undercover. The trail points to the Irish mob and a mole, who tie in trough love. Meanwhile Castle decides after long hesitation to read the message to himself he warned against and next to tell Becket it points to her nemesis.
Did You Know?
When Castle and Beckett are nearly caught by his mother at the beginning of the episode, a passageway into the hallway of the building seems to have appeared in Castle's bedroom out of thin air. And while it's possible Beckett snuck out through a window and navigated the outside of the building to an adjacent apartment, the time between when Castle notices she's gone and when she calls him is too short for her to have made it to where she did. See more
I'm surprised you're up, given how late you got in from LA last night.