The Boat That Rocked is a fictitious comedy set in Britain during 1966 in an era when the BBC as the only UK mainland licensed radio broadcaster (Manx Radio) had already been licensed to serve the Isle of Man), played little more than two hours of any kind of recorded music each week. In the story a pirate station called Radio Rock began broadcasting rock music twenty four hours a day from a boat anchored off the coast of England in international waters. Hosted by a colourful band of disc-jockeys they soon gain an audience of millions and anger the government in the process. While the story does has some relationship to real events it does not represent any specific station that was broadcasting to Britain in 1966.

Carl (Tom Sturridge) arrives on the pirate radio ship, Radio Rock, after being sent to stay with the ship's Captain, his godfather, Quentin (Bill Nighy), to hopefully set his life on a different track after being expelled from school. Here he meets Radio Rock's crew of ramshackle disc jockeys, led by The Count (Philip Seymour Hoffman), a buoyant rock-loving American, along with the suave and bawdy, Dave (Nick Frost) and the naive but good hearted, Simon (Chris O'Dowd). Also filling the airwaves is self proclaimed New Zealand "nut," Angus (Rhys Darby), the mysterious Midnight Mark (Tom Wisdom) and the even more mysterious and downright disillusioned Smooth Bob (Ralph Brown). Serving as the ship's crew are the shy lesbian, Felicity (Katherine Parkinson), and radio assistants, Harold (Ike Hamilton) and the appropriately nick-named Thick Kevin (Tom Brooke).

Dave wastes no time in introducing Carl to women, only for both of Carl's attempts to be foiled by the very man himself, including Carl's first crush, Quentin's niece, Marianne (Talulah Riley). Simon also is unlucky in love, only to meet the too good to be true Elenore (January Jones) only to find her affections are better placed with returning king of the airwaves, Gavin (Rhys Ifans). Gavin, the former star of Radio Rock, now has locked horns with The Count, both their egos, seemingly too big for the rusty wreck known as Radio Rock. The ship may have to set sail soon. Radio Rock's controversial on-air antics have ruffled the feathers of a government minister, Dormandy, (Kenneth Branagh), who takes down pirate radio, despite its popularity among the pop hungry masses. With that, Radio Rock sets sail for very stormy seas. The ship sinks. The minister gives orders not to save the crew, but fans do.

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