Plot: A new student, Terry Gross, is befriended and recruited by Bart and his gang (Milhouse and Nelson) to create mayhem at Springfield Elementary. Somehow, Principal Skinner remains a step-ahead of their pranks, leading Bart to suspect a mole in their operations (he wrongfully accuses Milhouse initially, before discovering it's been Terry- an orphan from Shelbyville Orphanage- the entire time.) Meanwhile, Homer falls in love with his rental car, and refuses to return it. The B-story of Homer and his rental are good for some laughs, but that is not what this episode is about. The main story - a parody of the Martin Scorsese film The Departed- features guest star Topher Grace of TV's That 70s Show as the mole in Bart's prank operation (the cooler and chairs set-up in Bart's treehouse mirrors the Foreman's basement from That 70s Show for an added touch.) The pranks in the episode reach a climax when Bart plans on combining gallons of Diet Coke with palettes of Mentos, which he succeeds in doing after deciding - mole or not - Terry is his friend. Unlike other episodes with guest stars, Grace's appearance doesn't hog the episode, but enhances Bart's character: his jealousy of another prankster; his mistrust of his friends; and his frank idiocy for not realising the new guy was the rat. Speaking of rats... Funniest Moment: Just like in The Departed, this episode ends with a rat, running across a fence. In case you missed what that stood for in The Departed, The Simpsons had resident simpleton Ralph Wiggum pop out of a nearby trash can and explain: "The rat symbolises obviousness."