"The Blunder Years" is the fifth episode of Season 13 (originally going to be an episode of season twelve, but was pushed to season thirteen due to the events of 9/11 back in the year, 2001).
This episode is also the last leftover from Season 12 in airing order after the tragic and awful events of 9/11, whilst "The Parent Rap" is the last episode of said previous season in season twelve's production order.
A restaurant hypnotist causes Homer to relive a traumatic childhood experience where, upon swimming in a quarry, he found a dead body. That corpse turned out to be Waylon Smithers' father, Waylon Smithers, Sr., who had disappeared twenty-five years earlier under suspicious circumstances. Meanwhile, Marge fancies Chad Sexington, the man who appears on the packaging of Burly paper towels.
Marge accidentally buys Burly paper towels and goes to return them when she "falls in love" with the lumberjack on them and finds that they are super absorbent. Homer becomes jealous and tricks Marge, making her believe that "Chad" is going to come for dinner. Marge prepares the house for his arrival, only to find out that "Chad" is actually Barney in disguise. Marge is humiliated once she realizes it is just a prank, which make Homer and Bart feeling guilty. Lisa encourages Homer and Bart to make it up to her with Homer taking them all to a posh restaurant.
At the restaurant, Marge admits that the prank was actually funny and it was the first prank that didn't go on fire. Soon a famous Indian hypnotist performs various acts of hypnosis, including making Professor Frink a makeout master, and when the hypnotist approaches Waylon Smithers and tells him that he will do something about his father, Smithers mysteriously replies he has no father; he died years ago. Dauntless, the hypnotist approaches Homer, and hypnotizes Homer into "turning into" various people, which since he has no idea who any of the people who he's been turned into are just result in him running around the stage saying "Out of my way!".
However, when the hypnotist turns Homer into his twelve-year-old self on summer vacation, he begins screaming hysterically, and still is screaming after being brought out of the trance, all the way home and even screams when he is asleep. The next day Lenny and Carlson bring him home from work because his screaming continued. Lisa says he is screaming because of a repressed memory. Marge makes Homer some Yaqui tea to unlock his suppressed memory.
After he drinks it, in a flashback to when he was twelve, he remembers going hiking with Lenny and Carl. They ran into a young Fat Tony, Legs, and Louie. They have blundered into their secret "wacky tobacky" patch. The bullies are about to punch and kick the hikers when a young Moe, shoots right in front of the toughs with a Daisy air rifle. The bullies run away. After the flashback ends, Moe reveals himself to be in the living room with Carl, Lenny, and the Simpsons.
They serve Moe Yaqui tea, and the Flashback continues. While camping, Moe, Homer, Carl and Lenny spot a large explosion at the Springfield Nuclear Power Plant which had recently opened and forget about it. Moe, Homer, Carl and Lenny go to an old quarry to have a swim. Homer was the only one dumb enough to jump in and found it was now a mud puddle. Moe assumes that Homer screamed because he fell in the mud, but Homer explains that he witnessed something else in the lake. At this point, in the present. Homer begins to remember what it was that caused him to start screaming. Wondering why the water was not coming out, he loosened the blockage and the water came out along with a decaying corpse that ended up in his lap. Homer started screaming endlessly (and hit puberty as a result).
In the present, the Simpsons visit the quarry, leaving the others behind. They find Chief Wiggum there and when Marge throws the Burly paper towels in, it absorbs the water where they find the corpse is now a skeleton. They go into the drainpipe and discover a hatch to Mr. Burns' office. Chief Wiggum accuses him of murder, but anticipating that this day would come, Burns shows a old reel film proving his innocence. It shows that the body was Waylon Smithers, Sr., who actually died saving the town and his son from an imminent nuclear meltdown. After the film, Burns confesses that he threw Smithers Sr.'s body down the sewer pipe since cover-ups were 'all the rage' back at the time and that he took the young Waylon Smithers Jr. under his wing out of dear empathy for Smithers Sr., who was considered to be a close friend. Burns also admits that he never told Smithers the truth about how his father died. However, Smithers sees the movie the entire time, realizing the truth. Burns apologizes to Smithers for giving false information to him, saying that he only did it to spare him the trauma of his father's loss. However, Smithers admits that he is grateful that his father died a real hero instead of being killed by a tribe of savage women (as told by Burns during his youth).
With the case behind the quarry solved, Homer happily stores the skull of the corpse inside his "Memories" box, despite Marge insisting that they give it to Mr. Smithers, to which Homer replies that he would just bury it again. However, Moe comes over with clues, and relates them to Homer and Marge all the way through the end credits, which finish with Homer screaming again during the Gracie Films and Fox logos.