Set in a surreal universe that mirrors a vast video game environment, the film begins in Toronto, Ontario, where we meet Scott Pilgrim (Michael Cera) as he enjoys time with his bandmates from Sex Bob-Omb. Among them is the drummer, Kim Pine (Alison Pill), who shares a high school history with Scott, along with Stephen Stills (Mark Webber) and Young Neil (Johnny Simmons). Scott is excitedly chatting about his new girlfriend, Knives Chau (Ellen Wong), a 17-year-old high school student who is five years his junior. Their relationship feels uncomplicated to him.
When Knives arrives to support them during practice, she becomes infatuated with the band. After the rehearsal, Scott walks her back home but avoids inviting her inside, indicating it's a "no girls allowed" zone. Instead, he shows her the house he grew up in, which is just across the street. Once back in his apartment, he confides in his gay roommate Wallace Wells (Kieran Culkin), admitting to dating a high school girl and asking him to keep it a secret. However, their conversation is cut short as Scott's sister, Stacey (Anna Kendrick), calls to scold him for his choice in partners—she heard the news from Wallace.
That night, Scott experiences a strange dream about a girl with hot pink hair roller-skating past him, telling him he’s merely dreaming. Alarmed, he shares this with Wallace, who sleeps beside him, as well as Other Scott (Ben Lewis), Wallace's boyfriend.
The next day, Scott meets Knives at the local library but finds himself distracted by thoughts of the mysterious pink-haired girl. While there, he spots her delivering a package and roller-skating away. At band practice later, his mind continues to wander. Stephen announces their entry into a battle of the bands, with their rival being Crash and the Boys.
Later that evening, Scott joins his friends at a house party hosted by Julie Powers (Aubrey Plaza), which he finds dull. He encounters Comeau (Nelson Franklin), who shows him a sketch of the girl he's been dreaming about—it's Ramona Flowers (Mary Elizabeth Winstead). After asking around, he learns that many attendees see her as "hardcore" and difficult to date. When he finally meets Ramona, he makes a clumsy attempt to flirt with her. Even after realizing he’s blown his chance, he can’t shake his fascination.
In an effort to get closer to Ramona, Scott tries to send her a package through Amazon, despite receiving a strange email about a duel with a 'League of Evil Exes,' which he mistakenly disregards. When Ramona eventually delivers the package, he awkwardly asks her out until she finally agrees. Their date, however, leads to a complicated mix of emotions as they delve into their pasts and insecurities.
Their adventures continue through various confrontations, culminating in a hilarious battle of wits and fights against Ramona's exes. Each duel is marked by unique challenges, revealing deeper character growth for Scott as he learns about love, fidelity, and self-respect. As he faces each of Ramona's seven ex-lovers, he uncovers his flaws and the motivations behind his feelings—landing him in chaotic yet revealing situations.
Ultimately, Scott's journey is one of self-discovery as he grapples with the tumultuous world surrounding him, leading up to a climactic showdown with Gideon Graves (Jason Schwartzman). Just before the culmination of his battles, Scott comes to terms with his mistakes, leading him to a moment of clarity where he understands who he truly fights for. In the end, Scott's choice to pursue an honest relationship with Ramona takes them through a whimsical final scene where they walk through another subspace door together, symbolizing a fresh start.
