Don Cliff Richard and friends Cyril Melvyn Hayes, Steve Teddy Green and Edwin Jeremy Bulloch are bus mechanics at the vast London Transport bus overhaul works in Aldenham, Hertfordshire. During a miserably wet British summer lunch break, Don arrives, having persuaded London Transport to lend him and his crew an AEC Regent III RT double-decker bus (which, throughout the film, still purports to be a route 9 to Piccadilly).
They convert the bus into a mobile home and drive across to continental Europe, aiming for the South of France. On the way, they are joined by a trio of young women, Sandy [Una Stubbs], Angie [Pamela Hart] and Mimsie [Jacqueline Daryl], who form the singing group Do-Re-Mi, and they decide to head to Athens in Greece instead. Their first stop is Paris, where they visit a nightclub where Les Shadows are playing. That night they discover a stowaway “boy,” Bobby, who is really Barbara [Lauri Peters], a runaway girl fed up with her overbearing American mother. Next they encounter a mime artist Orlando [Ron Moody]. Only after agreeing to give him a lift does he reveal that he has a large entourage to also be accommodated. Barbara’s mother works out where she is and arranges for the bus to be stopped, and the boys appear in court for carrying passengers. The judge agrees that if they can prove they are all entertainers they can go, so they put on an elaborate mime show. The show ends in an explosion and they escape, however Barbara is revealed as a girl. The mime troupe depart.
Mrs Winters and a male driver-friend Jerry [Lionel Murton] catch up with the bus in Switzerland. The bus struggles on the steep rough roads. Everyone gets off except Don driving and Barbara as a passenger to lessen the weight. The others walk, encountering a St Bernards dog, which they decide to keep. Mrs Winters and her friend follow the bus into Austria. They stop at Hotel zum Schloss and have a nice meal and pair off to dance. Don and Barbara go off to be alone and he admits that he loves her. Meanwhile Mrs Winters and her friend hide a diamond pendant on the empty bus and inform the Yugoslav authorities that it has been stolen. The bus is stopped by the border guards. The guards find it, but as it has a locket containing Barbara's picture they give it back.
Despite the efforts of Mrs Winters, a shepherdess in Yugoslavia puts them back on the right road, after taking them to her village for bread (they think). But there has been a mistranslation and the village thinks Don said “bride” and a marriage ceremony is begun. The boys eventually escape and run back to the bus pursued by armed peasants.
When they reach Athens a large crowd greets them due to media coverage. Wrightmore [Nicholas Phipps] from the British embassy explains the situation. Mrs Winters arrives. She first wants them arrested then she sees the publicity potential. Don goes off alone to clear his head on a hill overlooking the Acropolis. Mrs Winters locks Barbara in her hotel room while she holds a press conference. Don finds Barbara and asks her to marry him. The team help her escape through the window over the door. At the press conference Barbara announces their engagement and Don seems set to start a holiday firm operating 200 buses so all is forgiven.
