Wreck-It Marshall Lee is a movie from Cartoon Network that star all the Adventure time cast as some video game character. The film tells the story of the titular arcade game villain (Marshall Lee) who rebels against his role and dreams of becoming a hero. He travels between games in the arcade, and ultimately must eliminate a dire threat that could affect the entire arcade, and one that Marshall may have inadvertently started.
At night, when the Arcade closes, the various video game characters congregate in Game Central Station through the power cables. At the game Fix-It Marshaleo, the characters celebrate the game's titular hero but shun its villain, Wreck-It Marshall Lee. At a support group for video game villains, Marshall reveals he doesn't want to be a bad guy anymore. Returning to his own game, Marshall finds the other characters celebrating their game's anniversary and that he was not invited. Marsh reluctantly invites Marshall to join them, but the others shun him, saying he'd have to win a medal, just as Marsha does. While visiting Bob Rainicorn's, Marshall meets a soldier (Marsha's cousin) from the first-person shooter Heroic Duty who tells him the game's winner receives a medal. Marshall enters the game and encounters Sergeant Bubblegum, its no-nonsense leader. Between games, Marshall climbs the game's central beacon, which happened to be filled with Cyber-Bugs (Bug-like enemies) eggs, and collects the medal. However, he accidentally hatches a Cyber-Bug that clings to him. During his panic to get the Cyber-Bug larva off, he stumbles into a nearby escape pod that launches him out of the game. Meanwhile, Marshall's absence has not gone unnoticed, as a girl outside the game tells the owner that Fix-It Marshaleo is malfunctioning. Lady Rainicorn travels to Fix-It Marshaleo, and tells him about the dark turn of events. This is alarm for Marsh, because if a game becomes broken, the owner will have to unplug the game, leaving any characters in the game homeless. So, Marsh sets out to find Marshall.
Marshall crash-lands in Candy Kindgdom Rush, a kart-racing game. As he searches for his medal, he meets Fionna, a glitchy character who makes off with the medal, planning to use it to buy entry into an after-hours race. However, King Candy and the other racers refuse to let Fionna participate, saying she's not really part of the game. The other racers pay her a visit while she is building her own cart and destroy it, disabling her from racing. Marshall witnesses this and scares the racers off. Marshall then berates her, but she insults him right back. This angers him to the point that he smashes a jaw-breaker in half. Because of his amazing strength, Fionna gets an idea. She explains to Marshall that the medal can be won back if she gets first place in a race. She then offers to make a pact that if he can help her win, she'll give Marshall the medal back. Marshall reluctantly accepts her offer. After making said pact, they head off to the cart bakery, where the racers make their carts, and they build a cart together. However, a security guard notices the unauthorized activity, and the authorities are sent out, along with King Candy. After a long chase, they give King Candy's party the slip by hiding in Diet Cola Mountain, an incomplete extra race track. It is discovered that this is Fionna's home. From there, Marshall teaches Fionna how to drive her new cart.
Back in Heroic Duty, Marsh meets Sergeant Bubblegum, who explains that the Cyber-Bugs don't know that they are part of a game and are completely ruled by instinct to consume and multiply. As the two search for Marshall and the Cyber-Bug in Candy Kingdom Rush, Marsh explains that he is searching for Marshall, who had probably "gone DP". When Bubblegum asked Marsh about this term, he then explains that a long time ago, a racing game called "Dark Prince Time" was the most popular game in the arcade. It starred a self-obsessed character named Dark Prince. One day, a Ultimate RoadBlasters arcade cabinet entered the arcade and gained more popularity than Dark Prince Time. Out of jealousy, Dark Prince traveled to the other game and attempted to take it over. He crashed into the main player of the new racing game and crashed both games, which led to both of them being unplugged. As Marsh and Bubblegum progress, they fix the pod Marshall was in and fly off to search for the lone Cyber-Bug. Marsh later falls in love with Bubblegum. However, Bubblegum's past comes back to haunt her when Marsh refers to her as a 'dynamite gal', something that her fiancé, who was murdered by a Cyber-Bug on their wedding day, would call her. Distraught, she forces Marsh to leave her ship. A heartbroken Marsh walks to King Candy's castle and meets Sour Candy, King Candy's assistant. He locks up Marsh after he realizes he should have locked up Marshall. After a long search, Bubblegum soon discovers the Cyber-Bug has laid hundreds of eggs underground.
Before the race, King Candy finds Marshall in the absence of Fionna and offers Marshall his medal, which King Candy has dug into the game's code to retrieve. The only condition was that Fionna couldn't race. When Marshall asks why King Candy and the other racers hate her so much, he explains he really doesn't. Fionna is a glitch, and so, this would cause her to act abnormally (such as teleporting and jumping around, sometimes through objects). If she won the race, she would become an official part of the racing roster. He goes on to explain that her glitching would give gamers the impression that the game was broken, and the game would be unplugged. While everyone else could be evacuated from the game, fionna could not leave, as she is a glitch. As a result, she would die along with the program. So, King Candy leaves Marshall with that, and exits. When Fionna returns and give Marshall a medal that says "You're my hero", he explains to Fionna that she can not race for her own good. She doesn't believe him, and she threatens to race on her own. Marshall stops her and hangs her by her jacket on a nearby candy-cane tree. He then reluctantly proceeds to destroy her cart into pieces. Fionna tries to stop Marshall but it was no use. She falls and runs away, sobbing that he "Really is a Bad Guy."
Marshall goes back to Fix-It Marshaleo, and sees that the entire game is deserted, save for one lone citizen, Ice Princess. IP explains that the game was set to be unplugged in the morning, and that everyone has evacuated. He then gives Marshall the keys to Marsh's old penthouse after Marshall explains that he didn't want to live in the garbage dump that he lived in before. Alone, Marshall goes to the balcony and throws his medal at the screen that sits above the game. This causes the Out-of-Order sign that previously covered the screen to un-stick and start to fall off, revealing the side of the Candy Kingdom Rush arcade cabinet in front of Marshall's game. He discovers that Fionna is on the cabinet and wonders why she is on the machine's art work if she is a glitch. Marshall suspects something foul at play and returns to Sugar Rush.
He comes across Sour Candy who has gathered up the remains of Fionna's kart in a trash bin. Marshall places him in his mouth as a form of torture until he confesses. Sour Candy explains that King Candy changed Fionna's code on purpose. When Marshall asks about his motives, Sour Candy says he doesn't know why, and in fact, no one knows why. He explains it is like this because King Candy locked up all the characters' memories. Sour Candy then reveals that Fionna and Marsh are locked up in King Candy's Fungeon (fun dungeon). Upon locating Marsh, Marshall begs Marsh to fix the wrecked kart so Fionna can race. He agrees to do so after discovering how hard of a life Marshall has had. After also freeing and making amends with Fionna, they start the race, and as the race proceeds, the hatched Cyber-Bugs attack Candy Kingdom Rush. Marsh, Bubblegum, and Marshall then battle them. When Fionna catches up to King Candy mid-race, Fionna's glitching interferes with King Candy's code, and reveals that King Candy is actually Dark Prince! Dark Prince rams his car into Fionna's, causing her to be dragged in front of the car while approaching a walled fork in the road. Fionna finally takes control of her glitching to escape from Dark Prince, who is afterwards consumed by a Cyber-Bug. The group attempts to flee the doomed game, but Fionna cannot pass through the exit due to her status as a glitch. Bubblegum says the game can't be saved because there is no beacon in the game; the beacon in Heroic Duty attracts and kills the Cyber-Bugs much like a Bug-Zapper.
Marshall, in a last-ditch effort, heads to Diet Cola Mountain, where he plans on collapsing its Mentos stalactites into the cola at the bottom, causing an eruption that would attract the bugs. While on top of the mountain, he pounds the mentos into the diet cola below from the top. Before he can finish, however, a fusion of Dark Prince and a Cyber-Bug attacks him and carries him above the mountain. Marshall breaks free and dives toward the mountain, hoping his impact will start the eruption. Seeing Marshall dive towards the mountain, Fionna in turn uses her glitching abilities with the goal of catching Marshall. Marshall breaks through the roof of the mountain, but before he is killed in the eruption, Fionna catches him in her cart. The eruption shines a bright light, which in turn draws all of the Cyber-Bugs, including the one that ate Dark Prince, to their destruction. Because characters who die outside their own game are unable to regenerate (according to Cake) this means that all the Cyber-Bugs and Dark Prince die permanently.
After the Cyber-Bugs are drawn to the beacon, Marsh restores the finish line, and Fionna crosses it, restoring her memory as the game's lead character, and restoring the ruins of Candy Kingdom Rush. The gamers favor her as a character, despite her glitches (it's likely her ability to teleport short distances is perceived by gamers as a special power, rather than a glitch). Marsh is very happy and is so excited that he kisses Bubblegum in her cheek, much to her delight, and Bubblegum kisses Marsh full on the mouth. Marsh and Marshall return to Fix-It Marshaleo in time to show the owner that the game works, and they also give Lady and Co. a new opportunity to work with Marshall and crew in a "bonus level", sparing it and also giving its characters a new respect for Marshall's work as the villain. These new additions make Fix-it Marshaleo more popular than it has been in years, the players calling it "Retro" which Marshall interprets as "Old but Cool!". Marsh later marries Bubblegum, with Marshall being the Best Man and Fionna as the Maid of Honor. To all of their relief, there were no Cyber-Bugs. The credits (featuring Owl City's "When Can I See You Again?") show Marshall and Fionna jumping across different games, such as Ooo Fighter where they destroy a car , and Ralph gets shocked in a match.