It's 1966 and British artists are producing some of the greatest rock and roll in the world, but nationally owned British radio won't play any of the music. To fill the void, various ships have positioned themselves off the coast and broadcast such tunes to half the population. The biggest may be Radio Rock, run by Quentin (BILL NIGHY) on an old fishing trawler where American deejay The Count (PHILIP SEYMOUR HOFFMAN) is the biggest star, having replaced his legendary British counterpart, Gavin (RHYS IFANS).
There's also Dave (NICK FROST) who's quite the ladies man despite his appearance or the fact that no women, save for lesbian cook Felicity (KATHERINE PARKINSON), are permanently onboard. The cool Mark (TOM WISDOM) is even more successful with the opposite sex when they're brought onboard, unlike Angus (RHYS DARBY) or Harold (IKE HAMILTON), while Bob (RALPH BROWN) is rarely seen by the others due to his early morning broadcast schedule.
John (WILL ADAMSDALE) is the news reader, Thick Kevin (TOM BROOKE) isn't the brightest bulb on the ship, and normal Simon (CHRIS O'DOWD) is getting married to Elenore (JANUARY JONES) who's going to live onboard with him. But the youngest guy on the trawler turns out to be Carl (TOM STURRIDGE) who's been expelled from school and whose mom (EMMA THOMPSON) has sent him to spend time with Quentin, his godfather. Realizing the boy needs to do some growing up, he arranges for the attractive Marianne (TALULAH RILEY) to be Carl's date, all as Carl wonders if Quentin might really be the father he's never know.
Back on shore, stuffy government official Dormandy (KENNETH BRANAGH) has made it his quest to shut down the broadcast pirates and in particular Radio Rock. Ordering his assistant, Twatt (JACK DAVENPORT), to do the dirty work, Dormandy is determined to pull the plug on the off-shore radio operation. Yet, as he becomes ever more determined, so do those on the ship, especially with the return appearance of Gavin who immediately becomes The Count's rival.