A group of radicalized young Muslim men from Britain set their sights on becoming suicide bombers. At the forefront is Omar (Riz Ahmed), a critical thinker who vocally opposes Western society and its imperialistic practices. Accompanying him is his foolish and simple-minded friend Waj (Kayvan Novak), the hot-headed Barry (Nigel Lindsay), a recent White convert to Islam with a volatile temperament, and the gullible Faisal (Adeel Akhtar), who hilariously attempts to train crows as bombers.
Omar, who is married to Sofia (Preeya Kalidas) and has a son, makes a living as a security guard. Determined to remain off the radar of security agencies, Barry suggests an absurd method: swallowing SIM cards. But Omar quickly debunks this idea, stating that the cards can still be traced. As their plot thickens, Omar opts to go to Pakistan for more rigorous training. However, he refuses to bring Barry along, deeming him a liability, which incites Barry's furious protest. In an attempt to assert control, Omar restrains Barry with jumper cables and departs.
Once in Pakistan, Omar and Waj attend a terrorist training camp, where Barry forcibly includes a hesitant new recruit, Hassan (Arsher Ali), whom he discovered at a panel discussion. Although Omar has an uncle who can connect him with extremist trainers, their training is jeopardized after Waj unwittingly carries a mobile phone, leading to a disastrous incident where Omar accidentally misfires a rocket, resulting in casualties among fellow jihadists. This incident does, however, bolster Omar's authority when they return to Britain.
Upon returning, Omar and Waj falsely claim they were sent for special training in Somalia, while Omar grapples with Barry's reckless choice of recruiting Hassan, who has a penchant for documenting everything. Their plans intensify when Faisal starts buying bleach to disguise his identity.
The group begins gathering materials to construct improvised explosive devices but faces discord over their target. Barry proposes a reckless attack on a local mosque to "radicalize the moderates," which Omar vehemently opposes, arguing that killing fellow Muslims undermines their cause. As tensions rise, Faisal dreams of a 9/11-style bombing and naively believes crows can be trained to deliver explosives. In a comic yet tragic turn, he ultimately fails in training these birds.
During tests of their explosives, the group faces detours including an oblivious neighbor, Alice (Julia Davis), who unknowingly wanders into their lair, causing paranoia among the group. They hastily move their volatile materials only to confront further disaster when their getaway car breaks down. Faisal's clumsiness results in an accidental explosion, leading to internal conflict and a temporary disbandment. However, they reconcile after Omar expresses remorse and resolves to target the upcoming London Marathon.
Disguising themselves in costumes—a Honey Monster, a rider on an ostrich, a Teenage Mutant Ninja Turtle, and an inverted clown—they prepare for their plot. As apprehensions mount, Waj contemplates the morality of their mission, yet Omar persuades him to proceed. In a twist of fate, Hassan attempts to alert nearby police but finds himself caught in the chaos when Barry detonates his bomb, resulting in tragic consequences for both Hassan and law enforcement.
In the frenzy, Omar realizes that he has misled Waj into a harrowing scenario he wishes to escape. Amid police aggression, he navigates through crowds, attempting to prevent further disaster. In a series of misunderstandings and explosive fumbles, Omar and Waj face the amalgamation of guilt, chaos, and the fallout from their misguided intentions, culminating in a tragic yet absurd ending that brings their story full circle—with the innocent bearing the brunt of their radical decisions.
