The series finale begins at the same place as the crew's first delivery, the Moon. Fry and Leela are walking down the street when the mascot seen in episode two of season one is making balloon animals and Fry buys Leela a balloon of herself. Bender is so excited. Bender cartwheels the way to Luna Park. The crew then boards an amusement ride. Unfortunately, Bender accidentally barfs up nuts and bolts, because he kept cartwheeling and jumping, jamming the ride's gears and sending Leela flying through the park glass and into space. The hole is sealed by the mascot. The Moon Patrol only finds her stuffed animal, thinking her dead. But Fry then discovers that Leela had sheltered herself inside the plush while surviving on the air of her balloon double. Later, at their apartment, Fry confesses to Bender that after that near-death accident, he decides he's going to propose to Leela.
The next day, the Professor shows of his latest invention: the time button. With it, he can go back ten seconds in time. Only the user(s) of the button or those hiding in an anti-chroniton shelter can recall what happened. As the crew gets in the shelter to watch, Zoidberg comes in to boast that he got a ten-dollar bill from a guy leaving in a taxi. Farnsworth then takes the bill before using the button and everyone glees from Zoidsberg's disappointment as he finds no money in his pocket. Leela asks if he can use the button to go back as far as she wants by pressing it over and over again. He claims that's impossible, as it takes ten seconds for the button to recharge. Fry then asks him if using the button can make a moment last forever. The Professor takes offense, stating that the button is to be only used for scientific purposes, as he pushes it again to watch Zoidberg's misery.
However, Fry steals the button so he and Bender can steal diamonds (without the proprietor knowing about it) to create a golfball-size engagement ring. He then prepares his proposal to Leela at Elzar's. Unfortunately, the giant clam carrying her diamond ring bites off her right hand. Fry panics, but then uses the button to prevent this. However, he doesn't want to hear Leela agree to his proposal until they have it at the Vampire State Building. But as he waits for her arrive, she is apparently tardy. Thinking that Leela had rejected him, Fry decides to commit suicide by jumping off the building. But as he is falling, he spots Leela arriving. Due to his continuous use of the button, his watch was the only thing going forward. He uses the button in an attempt to save his life but can't get back on top.
Back at Planet Express, the Professor urges everyone to get into his anti-time shelter. With the button being used, the universe is in a constant time loop. Naturally, Bender is quick to rat Fry out. Unfortunately, they can't leave the shelter without causing a duplicate paradox. Their past selves will be caught in the loop while their present selves will be shredded across the timestream. The Professor demonstrates by throwing the dollar bill out as the button is being used. So they run to the Vampire State Building while carrying the shelter, stopping every ten seconds. But when they get there, Farnsworth accidentally distracts Fry, causing him to lose the button and hit the ground, dying on impact. Leela uses the button to undo this, but accidentally shredding the Professor as he was outside the shelter. Fry then explains his predicament to Leela amidst a series of deaths. Realizing they need an exit strategy, Bender waits for an opportune moment and saves Fry's life with his airbag. But as Fry bounces off and everyone celebrates, he accidentally falls on the button and breaks it, causing space-time to halt, while Fry and Leela are the only ones not affected.
With all of eternity to themselves, Fry and Leela get married and explore the world, all the while being pestered by a mysterious glitter. When they reach old age, they return to the Vampire State Building, where Fry recites his intended proposal. Then suddenly, the glitter turns into a tunnel, in which the Professor emerges. Turns out, the button merely rotated into an orthogonal time that runs right angles to this one. He's been tunneling for years in search of the button. When Fry reveals he's been carrying the pieces, Farnsworth quickly repairs it and modifies the device so that he can reset everything to before he invented the Time Button, leaving them with no memory of any event afterward. Fry and Leela decide to give their life another go and they share one last kiss before the button is pushed for the final time.
EASTER EGG: In the first scene of the episode, you can see that sunken cart in the crater in which that appeared in the second episode of season 1 as the Planet Express Ship heads for the parking lot.