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"Beware My Cheating Bart" is the eighteenth episode of Season 23.
When Bart is forced to chaperone Jimbo's girlfriend, Shauna, to a movie, Shauna ends up developing feelings for Bart, resulting in a whole bunch of trouble with the bullies at school. Meanwhile, Homer buys a new treadmill, which comes with an online TV website, and becomes hooked to the show "Stranded".
During a day out in the mall, Homer decides to dump off both Milhouse and Bart at a children's movie to eat at the food court. Bart is forced to chaperone Jimbo Jones' girlfriend Shauna to a Jennifer Aniston movie. Both of them leave shortly after seeing the only part of the movie that appeals to Shauna (the man's ass) and they hang out in the mall. Bart and Shauna meet again per Jimbo's demands at a boutique were Shauna steals one of the products and puts it in her bag but sets off the alarms when she tries to leave and is caught by the security guard but Bart bails her out out of fear of being beaten up by Jimbo. She shows her appreciation by flashing her breasts in front of him, leaving him both traumatized and smitten with her. Both decide to enter a romantic relationship behind Jimbo's back and it is not long before Bart becomes the target of Jimbo's wrath. Both attempt to hide from a vengeful Jimbo at Comic Book Guy's store, while the bully decides to stake out Bart's home with help from an oblivious Marge, so Bart ultimately has to return home and face Jimbo's fury. Shauna breaks up with both of them ultimately and with some prompting from Lisa, Bart receives his due punishment (being held upside down from his treehouse to have the "fear of God put into him") from Jimbo.
Meanwhile, Homer is persuaded to buy a state-of-the-art treadmill from a crafty salesman. When Lisa shows Homer that he can access television shows wirelessly, he takes advantage of the machine and develops an obsession with watching an old television show called Stranded, instead of working out. Eventually Marge, in a fit of rage, gives away all the spoilers of the episodes Homer has not watched yet. He is initially furious at Marge and threatens to divorce her for ruining the episode, but a romantic evening planned by her resolves matters.