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"The Spy Who Learned Me" is the twentieth episode of season 23.
Homer and Marge go through marriage problems, and when Homer suffers a concussion at work and gets two months off, he imagines the secret agent Stradivarius Cain, to help Homer be a better man. Meanwhile, Bart learns a way to prevent Nelson from stealing his lunch money - giving Nelson Krusty Burgers until he is too fat to punch anyone.
On their date night, Homer takes Marge to an action film featuring the legendary fictional spy character Stradivarius Cain, but Homer's endless series of humorous shoutouts during the film earn him as much contempt from Marge as they do kudos from Lenny and Carl. Marge makes it clear the next morning she is still ticked off at Homer, and he ends up feeling crestfallen on the job—although he feels much worse after Mr. Burns accidentally drives into the ladder Homer was using to mend lights and Homer suffers a severe concussion. Mr. Burns reluctantly agrees with the concerns of Smithers (moral) and his lawyer (legal) and gives Homer eight weeks off with pay. When Homer goes home, however, Marge's disgust with him has been matched by the discontents Bart, Lisa, and Grandpa feel about other matters, so Homer plans to keep his paid leave a secret. He pretends to go to work every day and initially has a lot of fun, but soon returns to melancholy over the poor state of affairs with Marge. He is then stunned to see Cain appear to him as an imaginary friend, who will tutor Homer on how to be irresistible to Marge, starting with a lesson in confidence at a local restaurant that culminates with Homer winning over the lovely wife of a narcotic baron swearing revenge.
Meanwhile, a new smartphone-scanning policy at Springfield Elementary brings Nelson's lunch-money shakedown to a new level and leads Bart to plan revenge. When he watches Declan Desmond's muckraking documentary Do You Want LIES With That?, about the Krusty Burger's shady dealings and unethical food practices, he realizes eating nothing but Krusty's fast food will be quite bad for Nelson, and Bart gives his tormentor a free coupon book that leads to Nelson becoming horrifically obese and unhealthy. Lisa then takes Nelson to show his terrible fate at Krusty's, so Krusty offers Nelson free time with his personal trainer. Nelson ends up in great shape and ready to beat up more kids than ever, but Lisa notes "he is tough on nerds, but easy on the eyes" as Bart stares at her in disbelief.
Marge learns from Lenny that Homer was out on leave, and becomes enraged at him, only for Homer to completely defuse her rage by taking Cain's advice and telling Marge the truth (that he lied about going to work, and used the time to learn to be a better husband). When they head out for a night of dancing, Montana spots them and plans to kill Homer, but Homer uses a line about lovely eyes that leads to him being forgiven. Marge then asks Homer seductively how much more leave time he has, but Homer mentioned that his paid vacation finished one week before, and they just have to wait for the call from Human Resources.