Short version (Thanks James)
Scott Pilgrim (Michael Cera), a meek, 22-year-old bassist madly in love with the striking Romona Flowers (Mary Elizabeth Winstead), discovers that he must defeat her seven exes in combat. Though each challenger is progressively difficult, he finally defeats them all, and agrees to run away with Romona to start a new life.
Perfect sized version (Thanks Forger)
Scott fights his way through six of the seven exes, before learning that the seventh is the the music agent his band has been wanting to sign with. The rest of the band agrees to a contract, but Scott leaves, and Young Neil takes his place. Gideon also takes Ramona back with him.
Scott eventually confronts Gideon at his club twice--the first time, Scott dies when he hesitates in revealing to Ramona and Knives that he had been going out with both of them. However, the extra life he picked up from the previous battle allows him to try the events leading up to that again, and this time he confesses to Knives and Ramona immediately regarding his unfaithfulness. He also makes his peace with the rest of Sex Bob-omb, and concedes that Young Neil is a better bass player than he ever was. He and Knives then confront Gideon, and defeat him.
In the end, Ramona leaves Toronto, but Scott goes with her at Knives' urging.
Very, very long version: (Thaks Brian)
Scott Pilgrim (Michael Cera) is an average, quiet 22 year old from Toronto. He plays in a band called the Sex Bob-ombs with his talented friend and lead guitarist Stephen (Mark Webber), who longs for the dream of the band being signed to a major record label, and his somewhat morose and apathetic friend and drummer Kim (Allison Pill), who makes is a point to note their band's obvious shortcomings. Scott shares an apartment with his rather sarcastic gay roommate Wallace Wells (Kieran Culkin) and at the film's start has a girlfriend named Knives Chau (Ellen Wong) who just happens to be a 17 year old high school student. Despite Scott enjoying his time with Knives, which amount to listening to her talk about 'high school drama' and play arcade games together (a game similar to DDR where movement translates to ninja attacks, important later), the obvious nature of the relationship comes to the delight/dismay of those who feel the need to drive this point home to Scott on a regular basis- namely Kim, Wallace, and Scott's social butterfly sister Stacey (Anna Kendrick). In an effort to warm his friends up to the idea of him dating her, Scott brings Knives to one of his band's practices. After an awkward introduction to Stephen's brother, "Young" Neil (Johnny Simmons), Knives hears the band play for the first time and becomes instantly hooked. The next day at the record store, Knives goes searching for CDs by The Clash at Demonhead, prompting an immediate dismissal by Scott of the band. We learn that the lead singer, Envy Adams (Brie Larson), was Scott's ex before Knives who left him to move away with someone and become a famous singer.
That night, Scott has a dream where he is wandering a desert when a mysterious girl with purple hair skates by on rollerblades. While at the library with Knives, Scott sees the mystery girl delivering a package to the librarian. At a party that night, Scott is informed that the girl is Ramona Flowers (Mary Elizabeth Winsted), and after briefing nearly everyone at the party is warned to stay far away from her as she contains "baggage". Frantically believing that she is his 'dream girl', Scott arranges for a package to be delivered to his house. During this, he receives an email from a man named Matthew Patel (Satya Bhabha) challenging him to a 'duel to the death', but dismisses it as spam and deletes it. Ramona promptly arrives at his house and reluctantly agrees to go out with Scott (he refuses to sign for the package unless she does). The two meet that night for their 'date', which despite Scott's awkward behavior and Ramona's cold demeanor ends with the two sleeping together (nothing happens, though). The next morning, Ramona gives Scott her phone number and agrees to attend a band gig that night. At the gig, Ramona meets Stacey, her boyfriend, Wallace (who flirts with Stacey's boyfriend all night, eventually winning him over), and Knives- forcing Scott to realize the conflict that he has yet to break up with Knives yet 'slept' with Ramona. During their performance, a man screams Scott's name and attempts to attack him. Scott evades the attacks and the man reveals himself to be Matthew Patel.
Ex #1: Scott Pilgrim vs. Matthew Patel
Scott realizes the email wasn't a hoax, but is confused who Patel is and why he's fighting him. Patel reveals that he is Ramona's ex-boyfriend and is on a mission to kill Scott Pilgrim. The two engage in an arcade game-esque battle together, but despite his ability to conjure fire, Patel is defeated when Scott delivers a 64-hit combo/flying uppercut that shatters him into a flurry of quarters and awarding Scott 1000 points. On their ride back home, Ramona informs Scott of "The League of Evil Exes", and that in order for her to be with him he must defeat all 7 exes to truly win her heart. Scott accepts the challenge, half in a daze over the fact that Ramona admitted the two were dating. The next night, Ramona is coming over to the apartment and Scott asks Wallace to step out for the night. Wallace agrees, saying he's heading to the set of his favorite movie actor (and crush) Lucas Lee (Chris Evans), who happens to be filming his next movie in Toronto. The date progresses as usual: awkward. Yet, as usual, the two end up making out until Scott becomes self-conscious of his hairstyle and takes Ramona for a walk as an excuse to wear his hat to cover his hair (it's explained that Envy used to love his hair short, and he subconsciously believes one of the reasons she dumped him is because it got too baggy). The two meet up with Wallace as Lucas walks onto the set, much to Ramona's disdain. Scott notices the number 2 on Lucas' trailer, jacket, shirt, etc. As he realizes the truth, Lucas (while filming) calls out Scott Pilgrim and knocks him out cold. As Scott gets up and tries to confront him, Lucas turns around and it's a stunt double. Suddenly several more appear and the fight begins....
Ex #2: Scott Pilgrim vs. Lucas Lee x 7
Scott engages Lucas' six body doubles, all armed with skateboards. Though he appears defeated, Scott emerges victorious and demands Lucas fight him one on one. Lucas agrees and proceeds to beat Scott senseless. Scott then challenges Lucas to do a 'grind thingy' down a steep rail covered in ice. Lucas accepts and grinds the rail successfully, but gains too much speed and- seen in a distance- collides into a brick wall at the bottom and explodes into more change, rewarding Scott 2000 more points. Scott goes to be with Ramona, but she left during the fight. Many days pass without hearing from her and during this Scott makes the decision to break up with Knives. This sends Knives into a frenzy, who blames Ramona for stealling Scott from her. Knives dyes her hair blue and starts going out with "Young" Neil to make Scott jealous, as well as becoming a die hard fan of The Clash at Demonhead. When Scott tries to explain his situation to his sister, he is attacked by a flying shadow. Scott punches the shadow hard, revealling herself to be a girl named Roxy (Mae Whitman). Roxy challenges Scott to battle, but Scott asks for her to come back another day because he's not in the mood. Roxy obliges and Scott ends up running into Envy. As it turns out, her band is in town and has decided to allow Sex Bob-omb to be her opening act for their performance. Ramona finally shows up and, after encouragement from Scott that he still wants to be with her despite the turmoil going on, goes with him to the show. During the performance, Scott points out to Ramona that the basist in The Clash at Demonhead is the "Todd guy" Envy left him for. Ramona once again gets upset and Scott starts to groan. Todd (Brandon Routh) has a #3 on his shirt. After the performance, Envy invites everyone backstage where tensions run high. Envy ridicules Ramona while Todd ridicules Scott. When Knives repeatedly interrupts Envy to tell her how much she idolizes her, Todd punches Knives and knocks her out. Enraged, Scott goes to attack Todd but his held back by an unknown force. Todd's eyes glow white and he telepathically shoves Scott through a brick wall. Todd explains that his power comes from being a pure Vegan, allowing him to use 100% of his brain (because the other 90% humans dont use is filled with curd). Scott continues to try to attack, but is thrown back again through another wall.
Ex #3: Scott Pilgrim vs. Todd Ingrim
Music chords begin appearing through the opening in the wall, and Scott challenges Todd to a "bass off". Scott puts up a fight, but Todd ends up victorious and destroys Scott's guitar. Before Todd delivers the finishing blow, Scott offers him a final latte for the two to share. Todd accepts and Scott reveals that he put half-and-half in the latte, breaking his "pure Vegan". A police car breaks through the wall and the Vegan Police (Thomas Jane) comes in and strips Todd of his powers since he violated the code of being a Vegan. Scott seizes the opportunity and headbutts Todd, shattering him into coins and rewarding him with 3000 points. Scott tells Natalie (Envy's real name) that since she broke his heart, that they're now even and leaves with Ramona. Both head to an after party where Scott and Ramona begin fighting over how many "ex-boyfriends" she has slept with and how many are coming to kill him, to which she keeps correcting him in saying "exes". When Scott asks why she keeps doing that, Roxy returns and attacks Scott. Sure enough, Roxy was an ex as well. Roxy is about to deal the finishing blow when Ramona stops her.
Ex #4: Ramona Flowers vs. Roxy Richter
Ramona produces a giant hammer from her purse, while Roxy removes the belt holding her clothing together to use as a razor sharp whip. The two battle fiercely before Roxy reminds Ramona that the rules state Scott has to be the one to defeat her and, as such, Ramona can't kill Roxy. Scott refuses to hit a girl, so Ramona grabs him from behind and uses him as a puppet to fight Roxy. The two gain the upper hand until Roxy teleports between the two, separating them. As Roxy is about to kill Scott, Ramona shouts to him her weak spot is behind her knee. Right before she can kill him with an axe kick, Scott pokes Roxy behind her knee, causing her to fall down in an orgasmic trance. She "climaxes" in an explosion of coins, rewarding Scott 4000 points. Ramona leaves the club, but before she does hands Scott a list of all the exes. He learns that #5 and #6 are the Katayanagi Twins, and incidentally are going to be at the next band gig. When the band arrives, they realize to their horror that they aren't opening for the Twins. They are battling them in an "Amp vs. Amp" band battle. As Stephen is freaking out about what's about to occur, Scott notices Ramona in the audience with a man. Stephen says that he is Gideon Graves (Jason Schwartzman), the "G-Man" that Stephen alluded to throughout the film as the record exec who would sign them. Scott recognizes the name Gideon as #7 on the list. This further encourages him to rally the band together for their fight.
Ex #5&6: Sex Bob-omb vs. the Katayanagi Twins
The battle is a music fight, and as Sex Bob-omb gains the lead in the battle the Twins turn their amps up to 11 and summon two large Chinese dragons made of pure music. The dragons blast Sex Bob-omb, knocking them backwards and seemingly ending the fight. Scott refuses to give up and, summoning everything they have, the band regroups and summons a large monster also made of music. The monster grabs both dragons and slams them together, the force of which causes the massive amps to fall on both Twins- killing them and rewarding Scott 11000 points and a 1-up. Scott meets up with Ramona and awkwardly tells her he loves her, to which she says she's leaving for New York City to be with Gideon again. Gideon signs Sex Bob-omb to his label (minus Scott, who is replaced by "Young" Neil) and drives off. Days later, Scott receives a call from Gideon who has decided to build one of his elaborate clubs in Toronto to deliberately taunt Scott in that he now has his band (who realize that in selling out, they are reduced to being a club band for life) and his girl. Scott suits up and arrives at the club, making it past both sets of guards all the way to Gideon. Scott tells him that he's doing all this because he's in love with Ramona, causing a large sword handle to protrude from his chest and him "levelling up". Scott uses the sword to battle the guards successfully.
Ex #7: Scott Pilgrim vs. Gideon Graves
Scott rushes at Gideon to attack him but is overpowered quickly. Knives shows up weilding two large daggers and begins fighting Ramona for stealling Scott. Scott subdues Gideon and breaks up the girls fighting, awkwardly revealling to both that he cheated on each of them (he slept with Ramona while with Knives and didn't tell Ramona he was dating Knives). Gideon uses the opportunity to stab Scott and kill him. Scott wakes up back in the desert, with Ramona appearing to him. She reveals that Gideon has a mind control chip implanted in her and that's why she left him, and that he needs to fight for a different reason than just for her. Scott has an epiphany and suddenly the 1-up takes effect, rewinding back before he enters the club and starting over. Enraged, Scott kills both guards. He then reassures the band that they're amazing, "Young" Neil that he's is truly worthy of replacing him, and apologizes to Kim for everything. He then tells Gideon that he's not fighting the League for Ramona, but for himself. A new, more powerful sword of "self-respect" appears and Scott levels up once again. Sex Bob-omb rally behind Scott as he destroys every guard without trying. Scott calls out to Knives knowing she's hiding and assures her that because he cheated on her, none of it is Ramona's fault. Knives forgives both of them and joins forces with Scott.
Ex #7: Scott Pilgrim & Knives Chau vs. Gideon Graves
The two severely weaken Gideon, but he continues to battle. The battle quickly shifts to a re-enactment of the ninja DDR game the two played and, using their skills as a duo, they successfully defeat and kill Gideon- rewarding both with 7,000,000 points. The chip in Ramona is deactivated but as the three go to leave, Gideon's image appears in the lens of his glasses telling Scott that there's one more challenge to defeat. A figure walks out from behind the three...
Final Battle: Scott Pilgrim vs. Nega Scott Pilgrim
Knives explains to Ramona that, like in their game, the final boss is the person's own 'negative version'- a personification of their violence and evil. Scott tells the two to leave and that he has to fight this on his own. As Knives and Ramona wait outside, the club door opens and both Scott and Nega Scott walk out talking to each other. Nega Scott walks away and Scott tells the two that because they have so much in common they decided to just become best friends instead. Ramona says goodbye to Scott and walks away, but Knives encourages him to go after her, saying that he's fought so hard for her. Scott catches up to Ramona and the two walk away hand in hand to start a new life together.