Brian and Charlie (played by Eric Idle and Robbie Coltrane) are petty thieves who work for a London gangster named "Casey." Casey tasks them with robbing a group of Chinese Triads who are dealing drugs in the city. After successfully stealing the profits, Brian and Charlie decide to leave the criminal world behind and keep the money for themselves.
However, Casey discovers their plan and sets out to have them killed. Brian and Charlie steal a car and speed away with the money, but are quickly pursued by Casey's henchmen. After a high-speed chase, they crash the car and are forced to escape on foot. Seeking refuge, they stumble upon a Catholic convent disguised as nuns.
Desperate to avoid being discovered by the gangsters, Brian and Charlie agree to hide out in the convent and pretend to be nuns. They're instructed to attend the teacher training program at the convent, where they struggle to adjust to their new lives and keep their identities secret from the Mother Superior and the other nuns.
As they spend more time at the convent, Brian and Charlie learn about the struggles of the nuns and the importance of their faith. However, things take a turn when a pair of real gangsters disguised as priests arrive at the convent looking for Brian and Charlie.
Brian and Charlie must navigate a series of comedic hijinks to maintain their disguise and avoid being caught, all while trying to find a way to escape with their stolen money. Along the way, they learn valuable lessons about friendship, faith, and the importance of doing the right thing.