A typical British summer at the seaside, as holidaymakers struggle in a constant battle with the rain. At a London Transport bus repair works, three young mechanics, Cyril, Steve and Edwin, discuss the gloomy prospect of a British holiday only seven days away, but Cyril hints that their mate Don has 'something up his sleeve'. Ever-cheerful Don arrives in his red double-decker bus, and tells them that LT have agreed to loan the bus for a continental summer holiday, provided they and their workmates re-equip it.
Seven days later, with the bus re-fitted, the four lads set off. On the road, they see a car with a dangerous wheel, and while warning the driver, are forced off road by an oncoming truck. The car is occupied by three girls, Sandy, Angie, Mimsie - a singing trio called 'Do-Re-Mi', on their way to a booking in Athens. As their motor is a write off, they eventually agree to accept the offer of a lift on the bus.
On their journey through France, as they pass over a railway crossing, they hear movement on the top deck. The boys find a nervous stowaway boy (who introduces himself as 'Bobby', aged 14), but his behaviour is rather strange when he is with the other boys.
They stop in a small town where Bobby buys a newspaper from a vendor's stand. Bobby (as missing singing heiress Barbara Winters) is on the front page. From the kiosk, Bobby makes a phone call to his mother Stella, (who asks her maid to trace the call) but refuses to divulge his location. The call is traced, and the proprietor tells the maid that she is travelling by bus to Athens. Stella is reminded by Barbara's agent Jerry that this is the kind of publicity which money cannot buy and should make Barbara world famous.
A man stands in middle of road and requests a lift from the travellers. He is a mime artist, 'The Great Orlando'. After they offer a lift, his company emerge from bushes. The bus is stopped by the police (tipped off by Jerry), and the gang appear before a magistrate, who is so impressed by their skills as mime 'entertainers' (under Orlando's guidance) that he releases them. But at the end of the show, Bobby's Pierrot costume is torn, revealing a pink slip underneath. Her disguise broken, Barbara offers to leave, but the gang insists she travels with them.
In Switzerland, Stella and Jerry catch up with the bus and alter a road sign so the bus is stuck on a hill. Don starts to notice Barbara, and arranges time so they can be apart from the others. As a way of saying 'thank you' to the boys, the girls and Barbara arrange an evening of music and dancing for them all at a 'White Horse Inn' type hotel with gardens. After dancing, Don takes Barbara into the gardens and tells her he is in love; they kiss in moonlight.
Stella and Jerry, meanwhile, plant a diamond pendant on the bus and report it to the Yugoslav border police as stolen. The guards find it, but Barbara insists it is hers, showing them her photo inside. The boys' search for 'bread' leads to a misunderstanding with a Yugoslav shepherdess, who thinks they want her as a 'bride'. In disguise, they escape from the pre-nuptial celebrations in the nick of time.
Arriving in Athens, they are met by the press and by the British consul representative, with Stella and Jerry in pursuit. Don learns from a newspaper that Barbara is a missing heiress, and feels betrayed. The consul rep reads them a cable informing them that London Transport will back their enterprise for a programme of tours commencing in two week's time. Stella has arranged her big press conference, which Don interrupts with his 'Big News'. Barbara declares her love for 'the hoodlum' Don, and announces that they are to marry. Stella is also won over by Don's bold business plans. In the meantime, the boys and girls enjoy the holiday they came for, as their bus arrives at the coast and they take their first dip in the sea.
