On a trip to the extremely rundown Springfield Mall (Bart says that this is because Mayor Quimby's father was murdered there), very similar to the Dixie Square Mall, Homer happens across the Time–Life Carpenter's Library, and Marge encourages him to buy them. Homer's interest in carpentry fades, and Marge decides to use them herself to fix up the house, starting with her broken nightstand. She begins to learn more and more about carpentry and Lisa suggests to Marge that she try to earn some money as a handyman, thus opening up 'Simpsons Carpentry'. However, potential clients (Superintendent Chalmers and Krusty the Clown) turn her down, dismissing the idea of a female carpenter. Commenting on how people expect carpenters to be male, Marge develops a plan. She uses Homer as a front to the customers, while Marge, hiding in an accompanied red tool chest, does all of the work as Homer rests in the tool box, and switching back when the customers come to check on Homer's work. Meanwhile, a note is sent out from the school, informing parents that someone at the school has a peanut allergy so foods containing peanuts will no longer be allowed on school premises. An indignant Bart claims it to be unfair not to disclose the identity of the "kid", but he soon discovers the "kid" is actually Principal Skinner. With this newfound knowledge, and with the assistance of a peanut on a stick, Bart forces Skinner to do his bidding (publicly humiliating and injuring himself).
While business is going great, when she is out buying supplies Marge becomes a bit discouraged by Helen Lovejoy and Lindsay Naegle, who taunt her for being Homer's "killer". That night, Marge tells Homer that she feels he is taking too much credit and wishes she would get some recognition for the work. Homer, however, does not want to be humiliated by revealing his wife did everything. After an incident in which Homer mocks Marge's carpentry skills with Lenny Leonard and Carl Carlson and then tells her he has been hired for the biggest commission yet in repairing Springfield's old wooden roller coaster, "The Zoominator" but expects her to do all the work and let him get all the credit, Marge angrily tells him she quits and he's going to have to do the work by himself. Homer tries to fake his way through being a foreman in front of his newly hired construction crew, but they eventually abandon him when he reveals he cannot pay them and does not know anything about construction. Meanwhile, after being advised by Comic Book Guy that the only way to stop Bart is to find his "kryptonite", Skinner breaks into the Springfield General Hospital and searches through medical records of Bart in the night. He finds out that Bart is allergic to shrimp. The next day, Skinner counters Bart's peanut stick with his own: a shrimp on a stick. Bart and Skinner clash Star Wars style with their respective "sticks" with "Duel of the Fates" played in the background. They eventually get led into a Thai food factory in the "Little Bangkok" section of town. They battle over a rickety catwalk, which is straight over top of a vat of shrimp-peanut mixture. Skinner sees this and attempts to end the battle, but Bart, claiming that 'kids don't die', rushes at Skinner, causing both of them to topple into the vat, and putting them in the hospital for their allergic reactions.
The big reopening day arrives, and Homer stands in front of a crowd gathered to witness the unveiling of the refurbished roller coaster. Marge has a video camera at the ready, so she can catch Homer being exposed as a fraud on tape. When revealed, the crowd is in awe at the seemingly repaired roller coaster, but with a hit from Homer's popped cork from his champagne, the coaster starts to break, becoming as rundown as it had been before. Still not wanting to admit the truth, Homer proves the coaster is safe by riding it himself, even though there are large gaps in the track. Acting quickly, Marge repairs each broken piece just before Homer's cart runs over it. While riding, Homer finally reveals to the crowd that Marge did all the handiwork. The crowd applauds as the coaster comes to a stop at the beginning, and just as Marge is about to tell Homer that she loves him, the entire roller coaster structure comes crashing down on top of Homer. At the hospital, Marge visits Homer, who is immobilized in a full-body cast, and he sweetly tells her that marriage is the real rollercoaster and he's thrilled to have Marge as "my safety bar". Bart and Skinner rest in their respective hospital beds in the same room as Homer; Skinner is outraged to hear Bart is pretending that he saved Skinner's life, and the two proceed to throw shrimp and peanuts at each other to set off their respective allergies again, but the reactions are just hives and a disgusted Marge decides to head over to the maternity ward observation room to see the new babies.