A Lamborghini Miura drives through the Italian Alps, enters a tunnel, crashes and explodes. A bulldozer pulls the remains from the tunnel and dumps them down a steep alpine gorge. Some time later dapper mobster Charlie Croker (Caine) is released from prison. He soon meets with the widow (Goldoni) of his friend and fellow thief Roger Beckermann (Brazzi), victim of the Miura crash. She gives Croker her husband's plans for the robbery that attracted the attention of the Italian Mafia. The plans outline a way to rob the payroll of Turin-based automaker Fiat, and spirit it out of Italy.
Croker decides to continue the plan despite the risks, but needs a large, well-equipped gang. He breaks into jail to meet Mr Bridger (Coward), a criminal who runs a gangland empire from prison. Croker explains "the Italian Job" but Bridger dismisses the plan out of hand, and indeed orders Croker be given "a good going-over" for disturbing his privacy.
Bridger changes his mind shortly after, when it is announced that China is delivering a consignment of gold to Turin, as down-payment to Fiat for the building of a car factory. With this backing, Croker assembles a group including computer expert Professor Peach (Hill), electronics handler Birkinshaw (Emney) and several getaway drivers. The plan calls for Peach to infect Turin's computerised traffic control to create a paralyzing traffic jam that will prevent the police from recapturing the gold. Three Mini Cooper Ss, able to navigate the gridlock in unconventional ways, will follow Beckermann's route through Turin to evacuate the gold.
After planning and training, Croker and crew set out for Turin. Mafia boss Altabani (Raf Vallone) is waiting at an Alpine pass with a front-end loader. It damages their two Jaguar E-Types and flips Croker's Aston Martin DB4 into the gorge, but Croker talks their way out of being killed by promising the Italian community in Britain will suffer reprisals if anything happens to them. He gathers the gang and has Peach load his guerrilla software into the traffic control computer the night before the heist. The next day Birkinshaw jams the closed circuit television that monitors traffic, just before Peach's software goes off and the city comes to a horn-honking standstill. The gang converge on the gold convoy, overpower the guards, pull the armoured car into the entrance hall of the Museo Egizio, and lock the doors.
Inside, the gang transfer the gold to the Minis. Mafioso Altabani recognises that "If they planned this [traffic] jam, then they must have planned a way out of it." The three Minis race through the shopping arcades of the Via Roma, up the sail-like roof of the Palazzo a Vela, around the rooftop test track of the Fiat Lingotto factory and down the steps of the Gran Madre di Dio church while a wedding is in progress. In a deleted scene, the police nearly capture the minis at a school where a band is performing the Blue Danube. The gang escapes by driving through large sewer pipes, throwing off the police. The gang make their final getaway on a six-wheeled Harrington Legionnaire-bodied Bedford VAL coach, driving up a ramp on the back while the coach is travelling. Once the gold has been unloaded, the gang push the Minis out of the coach as it negotiates hairpin bends in the Alps.
Charlie and the Mini crews meet the rest of the gang, who had sneaked out of the city disguised as English football fans in a minivan. On their way to Switzerland on a winding mountain road, the celebration grows raucous as beer flows. When driver Big William sends the coach into a skid, the back of the bus is left teetering over a cliff and the gold slides towards the rear doors. As Croker attempts to reach the gold, it slips further, and the audience is left not knowing whether the coach, its contents, or its occupants survivea literal cliffhanger. Croker's last line is "Hang on a minute, lads, I've got a great idea! Err..." The camera zooms out, still showing the bus on the edge of the cliff as the credits roll.