Kat Boyles, portrayed by Bryce Dallas Howard, is a struggling stand-up comedian and a part-time instructor at an improv acting school situated in London. One day, she is approached by Detective Sergeant Graham Billings, played by Sean Bean, who is seeking talented improv actors for low-level sting operations. To complete a team of three required for the job, Kat selects two of her recent students: Marlon Swift, represented by Orlando Bloom, an aspiring drama actor hunting for his big break, and Hugh, portrayed by Nick Mohammed, a clumsy IT worker struggling to fit in with his more humorous colleagues. Despite their reluctance, they are the only two options left.
Billings briefs the trio about the increasing sales of counterfeit cigarettes and instructs them to investigate a tobacco store rumored to be involved in these illegal activities. During their initial attempt, things take an unexpected turn when Hugh almost blows their cover. They are taken to meet Fly, played by Paddy Considine, the store's underground supplier. Kat cleverly convinces Fly that they are experienced criminals. As the investigation deepens, Fly asks them to help him dispose of three bricks of cocaine that his men recently stole from a rival Albanian gang. Things escalate further when the Albanians storm in, demanding their drugs. In a quick-thinking moment, Kat asserts that the drugs belong to them and hastily starts an auction, forcing the Albanians to unknowingly bid on their own stash.
Impressed by their bravado, Fly offers them a job, while Billings seizes the opportunity to enlist them as undercover agents to infiltrate and take down Fly from within his organization, assuring them that a tactical backup team would be on standby should the situation become dangerous. The trio adopts new identities: Kat as Bonnie, Marlon as Roach, and Hugh as The Squire.
Their first assignment leads them to a client named Sagar, played by Omid Djalili, who owes Fly money. After a tense confrontation, Sagar attempts to flee but is tragically hit by a van, resulting in his demise. The trio captures a photo of Sagar and seizes his cash just before the police are alerted. Dawes, the lead investigator, alongside his assistant Skender, mistakenly believes the trio to be a fresh wave of hardcore criminals since Sagar was notorious as a hardened assassin known as "the Iceman."
Fly and his associate, Shosh, are equally impressed by their alleged accomplishment and invite them to celebrate Fly's birthday. During festivities, Fly opens up about his personal life, sharing that his daughter is currently in Portugal. However, the celebration is abruptly interrupted when he receives a call from his boss, Metcalfe, who informs him that the Albanians are suspicious of the drugs they purchased from Bonnie. As tensions rise, Metcalfe instructs the trio to sample drugs from another dealer, K-Lash, but her crew is only interested in selling nitrous oxide, which is not illegal.
When Marlon, in character as Roach, tries to persuade K-Lash to sell them actual drugs, it ends in failure. Just as K-Lash's crew attempts to eliminate them, Billings arrives in tactical gear, causing them to flee. Frustrated by the situation's low stakes, Billings reveals his true colors, admitting that he is a corrupt cop exploiting them for personal gain. He blackmails them into complying with his plans until they can take down Metcalfe. However, his life comes to an abrupt end when he is killed by Shosh.
Shosh then brings the trio and Billings' corpse to Metcalfe, who orders them to dismember the body to hide evidence. After the disposal, they learn about the ongoing hacking of Billings' phone. While Hugh visits Shosh, Kat and Marlon attempt to retrieve the phone but are captured by Metcalfe's men, who are under the impression they are a legitimate threat following Sagar's death.
Realizing that their cover is compromised, they concoct a plan to approach Fly as undercover officers, intending to wire him to capture the drug deal involving Metcalfe and the Albanians. As the deal unfolds, the trio interrupts, securing Fly's safety while the police raid the operations. Just when they think they have evaded danger, Metcalfe confronts them at gunpoint but is swiftly killed by Shosh.
Realizing her predicament, Shosh bids farewell to Hugh, referring to him as "Squire" one last time. Fly, perplexed upon discovering the truth about their identities as mere improv actors, graciously hands each of them a hefty sum of cash from Metcalfe’s briefcase.
Dawes and Skender express their gratitude for the trio's unintentional assistance, which has led to significant busts within the criminal underworld. However, they can't acknowledge the trio's contributions publicly for fear of needing their help in the future. As the dust settles, Marlon auditions for a serious theatrical role, drawing inspiration from Fly's monologue, while Hugh gains newfound confidence and opens a wine shop. Meanwhile, Kat welcomes both new and returning students to her improv class, fueled by rumors of her undercover cop persona. Opting not to address these speculations, she dives straight into teaching once again.
