Set in a surreal world where our whole society is a giant computer system video game, the movie opens in Toronto, Ontario with Scott Pilgrim (Michael Cera) hanging out with his bandmates in a local rock and roll band called, Sex Bob-Omb. The drummer, Kim Pine (Alison Pill) used to go out with Scott in high school. Stephen Stills (Mark Webber) is The Talent and Young Neil (Johnny Simmons) lives there and hangs out with the band. Scott is telling them about his new girlfriend, Knives Chau (Ellen Wong). She's a 17-year-old high schooler five years younger then him. He likes going out with her because its simple. Knives arrives to watch them practice. As they practice, Knives watches in awe and becomes their biggest fan. After practice, Scott walks home with Knives. She's fawning and Scott eats it up. They arrive at his place but he doesn't invite her in. He says its a no girls allowed kind of place but asks her if she wants to see the house he grew up in. She says yes and they walk a couple feet across the street and there it is.
Scott comes into his apartment and tells his gay roommate Wallace Wells (Kieran Culkin) that before he hears any rumors, yes he is dating a high school girl and tells him not to tell her. Immediately after, Scotts sister Stacey (Anna Kendrick) calls and yells at him for dating a high school girl. She heard it from Wallace. Later at night, Scott has a dream where he's in the desert all alone when all of a sudden a girl with hot pink hair roller skates by and tells him that he's just dreaming. Scott wakes up alarmed and tells Wallace, who sleeps in the same bed, as well as Other Scott (Ben Lewis) Wallace's boyfriend.
The next day, Scott meets up with Knives at the library. He's distracted, as he can't get the mysterious pink-haired girl out of his head. As they head towards the check out counter, he sees her delivering a package and roller-skates away. Later, at band practice, Scott is still distracted. Stephen tells them the good news that they are going to be competing in a battle of the bands, where their main rival will be Crash and the Boys.
That night, they go to Julie Powers (Aubrey Plaza) house party to schmooze and hang out with record people. This is boring to Scott who declares he will now go pee out of boredom. Scott sees Comeau (Nelson Franklin) who knows everyone and shows him a picture that he drew of the girl he saw. Comeau says yes, that is Ramona Flowers (Mary Elizabeth Winstead), who is supposed to be at the party tonight. Scott goes around the entire party looking for her and asking about her. Many party guest say that she is hardcore and hard to date. He finally finds her and tries to hit on her, failing miserably. He says he will leave her alone now. Scott goes to Julie and asks what her deal is. She says she's American and has a job at Amazon. Stephen, who used to date Julie, says she just broke up with some Gideon guy in New York. Julie is angry because she didn't want Scott to know that. During the rest of the party, he stalks her until she leaves.
The next day, Scott goes onto the computer to order something from Amazon.com so Ramona will deliver it. He also gets an e-mail from some guy named Patel talking about a duel and a 'League of Evil Exes' but Scott declares it boring and deletes it. He sits in front of the door, waiting for the package he just ordered. The bell rings but its Knives. He hangs out with her begrudgingly but all he can think about is Ramona. He has another dream where he sees Ramona skating by and follows her to his own door. He wakes up and rushes to the front door and opens it before she can ring the bell. Ramona says there's a door in his subconscious that is a really easy shortcut in the subspace highway. She forgets that they don't have too much of that in Canada. Scott asks her out repeatedly and she finally relents.
Scott and Ramona go out for a date outside, despite the snow and cold. To escape the cold, Ramona takes another subspace shortcut door and they arrive at her apartment. They make out but Ramona decides not to have sex with him tonight. Scott stays the night. The next day, Scott invites her to the battle of the bands.
Scott is at the battle of the bands and sees Ramona. Also there, are Wallace, Stacey, and her friend Jimmy. Wallace constantly hits on Jimmy all night. Knives also arrives making it extremely awkward for Scott as everyone stares at him. He runs away backstage. Crash and the Boys play a great set, which freaks out Stephen thinking they can't follow that. They go up and in the middle of their performance Matthew Patel (Satya Bhabha) crashes into the room and attacks Scott. They get into a big fight. Scott asks him why they are fighting. He is one of Ramona's seven evil ex lovers. And by the rules of the video game world which they live in, he must defeat them all in order to go out with her. Scott defeats him and he bursts into a bunch of coins. Scott and Ramona leave on a bus where she explains that they are now dating but he must defeat all of her evil ex lovers. With one down, there are six to go.
Scott tells Wallace that he needs the place in order to have a date with Ramona tonight. Wallace tells him that he will let him as long as he breaks up with Knives right now. Scott doesnt want to because itll be too hard but he does it anyway. Knives tells Scott shes in love with him but Scott breaks it off. Wallace goes to The Castle to watch them film a movie starring ex-pro skater, now action star Lucas Lee (Chris Evans) while Scott and Ramona have their date. Their date goes fine but Scott starts to feel uncomfortable once the conversation turns to their pasts. Scott had a bad break up with a girl named Envy Adams (Brie Larson) who left him after her band Clash at Demonhead got a huge record deal. They go over to the Castle to watch the movie shoot and they find out that Lucas Lee was one of Ramonas evil exes. Scott gets beat up by his stunt doubles but he turns it around and beats them all up. But Lucas is too powerful so Scott goads him into grinding with his skateboard on a really long rail. He agrees and does it but he goes too fast and explodes, defeating him. Scotts sad because he didnt get his autograph. Ramona left during the fight without saying bye to Scott.
Scott tries to call her the next day but she won't answer. Wallace is rooting for their relationship so Scott can move out and he wont have to feel guilty about kicking him out. In an allyway, Scott is randomly attacked by a young teenage punk girl named Roxy Richter (Mae Whitman). He say's hes not really in the mood for this so she leaves, threatening to return later.
Scott goes to the coffee shop where Stacey works to talk to her but she's already left. Scott finds Ramona, however, and they talk about how they dont want their past to affect their current relationship. At that moment, Scott sees Envy standing behind her. Her band Clash at Demonhead is playing in Toronto tonight and she invites Scott and his friends, including Ramona to come by and listen.
At the concert, we find out that the bassist, Todd Ingram (Brandon Routh) is another one of Ramonas evil exes. We also find Knives is going out with Young Neil now, only to stay close to Scott. Knives notices that Scott is with Ramona now, which drives her to a jealous rage. Envy invites them all backstage where they very awkwardly sit together. Knives acts like an overexcited fan and Todd punches her. Scott pounces, ready to fight, but Todd has psychic powers and stops him mid-air. He has superpowers because he's a vegan and he went to Vegan Academy. They fight for a bit but Scott finds its hopeless so he challenges him to a bass battle. Todd wins knocking him into the next room. Before Todd can finish Scott off, Scott offers Todd some coffee with soymilk in it. But it actually has milk in it, forcing Todd to lose his vegan powers. Then the Vegan Police (Thomas Jane and Clifton Collins Jr.) arrive and punish Todd by destroying him. He bursts into coins. Scott apologizes to Envy and leaves with Ramona.
Scott and Ramona go to Clash at the Demonheads after party. Scott is upset that all of Ramonas evil exes keep trying to kill him. He feels threatened by her past. Roxy returns and punches Scott in the back of the head. This is another one of Ramona's evil exes. Apparently Ramona went through a lesbian phase for a few weeks. Ramona decides to defend Scott herself and pulls out a giant sledgehammer from her purse and fights Roxy. Roxy tells Ramona that according to the rules, it only counts if Scott fights her, but he refuses to fight a girl. Ramona takes him and uses his body to fight Roxy. They defeat her. Scott is upset about having to face off with all these exes and is thinking of leaving Ramona. She gives him a list of the rest of the evil exes he has to defeat and she leaves.
At the band meeting, the band tries to plan out what they need to do to win their next battle of the bands gig. They are going up against Katayangai (Keita Saito and Shota Saito), a pair of twins that Ramona dated before. At the concert, they have a huge band battle as Ramona watches with Gideon Graves (Jason Schwartzman) in the audience. The two bands form giant electric monsters that fight and Sex Bob-Omb wins, defeating the fifth and sixth evil exes. Scott grabs an extra life that pops up by his head. Outside, Scott tries to talk her back but she has decided to get back with Gideon. Gideon says he liked the band and wants to sign them. Scott wont sign with him but the rest of the band will with Young Neil taking Scott's place on bass.
Scott wallows in self-pity but Wallace talks him out of it and tells him to go fight for Ramona. Scott goes to the secret nightclub where Gideon is hanging out and where Sex Bob-Omb is playing. He arrives and tells Gideon he is going to fight him for Ramona because he loves her. He achieves the power of love and pulls a giant flaming sword out of his chest. Scott fights Gideon's minions and then Gideon himself. Knives arrives, saving Scott from Gideon. But Knives then challenges Ramona because she has broken Scott's heart. They start fighting but Scott stops them. He admits that he cheated on Knives with Ramona but doesn't think he cheated on Ramona with Knives. Gideon stabs Scott in the back and he dies.
Scott is transported to the desert, alone and dead. Ramona is there and they talk about what he has learned and who he is really fighting for. Scott uses the extra life, which puts him back outside to the beginning of the scene. He marches back in repeating the fight up until Gideon asks him who he's fighting for. This time Scott says 'himself' and he gains the power of self-respect and pulls out another giant sword from his chest. This time he launches himself at Gideon and defeats him. Scott calls out for Knives but she jumps down and kicks Ramona in the head. Scott stops her and admits he cheated on both of them and takes full responsibility. Gideon gets back up. Knives and Scott team up to defeat Gideon. They beat him and he dissapears into 10,000 coins.
Just before they leave, Nega Scott (an evil version of Scott) arrives. He tells Ramona and Knives to wait outside. He has to fight him alone. Outside, Ramona and Knives wait. The door opens and Nega Scott and Scott walk out laughing and conversing. Scott tells them that hes a really nice guy and theyre going to meet for brunch next week. Ramona leaves telling him that she doesnt want to make him another evil ex. Scott turns to Knives but she tells him to go after her. Shes the reason he went through all of this. Scott catches up to her and they get together. In the final shot, Scott and Ramona walk through another subspace door as the camera pans up into the sky.