"Dangerous Curves" is the fifth episode of Season 20.
The Simpsons take a road trip to a cabin in the woods that Homer and Marge once vacationed at 20 years before. In a series of flashbacks Homer and Marge reflect on some history from their courtship and Bart and Lisa recall some pleasant memories from their very early days.
On the Fourth of July, the Simpson family takes a road trip to visit a cabin in the woods. While driving there, they pick up hitchhikers Squeaky-Voiced Teen and his girlfriend, Beatrice. Staring at the two younger teens, Homer to flashes back to how he and Marge were, in their dating years.
In their dating years, a young Homer and Marge are on bikes, riding down the highway. Homer and Marge attempt to kiss while biking, but Homer crashes his and Marge's bike, prompting them to walk on foot, lost. Ned and Maude Flanders, who are driving by, notice them as hitchhikers and pick them up. Ned and Maude shock Homer and Marge by revealing the two of them are married (and since 2:00 pm that day); Ned attempts to convince Homer not to have any sexual activities with Marge, much to Homer's discomfort. Back in the present, Homer becomes quite annoyed with the Squeaky Voiced Teen's kissing Beatrice, which prompts him to go into another flashback, in Homer and Marge's married years.
In their married years, Homer and Marge are more stressed. After Homer is harassed by Patty and Selma, he stops and forces them out of car. Marge says that they have the map. When their car runs out of fuel, they head to a nearby home to borrow their phone. The house owner, Alberto, is having a party, and he invites Homer and Marge inside. Wishing to take a break, they accept. Marge becomes annoyed with Homer's behavior at the party, and becomes enraged after seeing him flirt with a beautiful woman named Sylvia. After Homer knocks Marge into the pool and starts a sushi fight, Marge regrets marrying Homer. In the present, the family drops of the Squeaky Voiced Teen and Beatrice at a cabin, while the Simpsons head off to their cabin.
Homer and Marge flash back to their dating years again, where the Flanders dropped them off at the very same cabins they are at today. Flanders manages to successfully convince Homer and Marge to sleep in separate rooms, much to their disappointment. In their married years again, Marge leaves Alberto's party with him to run to the cabins. Similarly, Homer runs off with Sylvia to those very same cabins. Not wanting each other to discover their own affairs, Marge hides Alberto in a box and has Homer put it outside; Homer, not knowing Alberto is in there, puts Sylvia inside as well. Alberto and Sylvia fall in love while Homer and Marge rekindle their love. In the present, Homer and Marge meet Alberto and Sylvia, now married with a child of their own, and learn of each others affairs. Disgusted that the most important moment in their early years of marriage was based on lies and deceit, Homer regrets marrying Marge and, trapped in a ball of their luggage which happened while unpacking, has Alberto and Sylvia's child roll him away from Marge into the woods.
Back in their dating years, Ned tells Homer that if he were married to Marge, he could make all the love to her he wants. Ned jokes to Homer saying that he has better things to do on his honeymoon than following about two teenagers, the next scene has Maude sitting in a tree with a water gun, and Ned telling Maude through a walkie talkie get Homer wiener locked. Taking a walk through the woods with Marge, Homer carves the message "Marge + Homer 4ever" into a tree. In the present, Homer sees this message on the same tree and thinks there is still time to save his marriage. He attempts to peel the bark off the tree and show the message to Marge; Marge suddenly arrives to find Homer and accidentally knocks the tree, which had severely weakened roots, over a ravine. Homer clings onto the bark and refuses to let go, but Marge tells him that their love for each other is within themselves, not in the bark. Homer falls down the ravine toward the river below, still holding onto the bark and peeling it all off the tree, with Marge falling after. They stop falling when the line of bark stops, allowing them to be saved by Bart and Lisa in their pedal car, which Bart had accidentally driven into the river. Then, the episode finishes with cars (and Homer tied to the ball) coming along the screen and splitting in halves.