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In 1966, numerous pirate radio stations broadcast to the United Kingdom from ships anchored in international waters, specialising in rock and pop music that is not played on BBC Radio. Seventeen-year-old Carl (Tom Sturridge), recently expelled from school, is sent to stay with his godfather Quentin (Bill Nighy), who runs the station "Radio Rock" anchored in the North Sea. The eclectic crew of disc jockeys and staffers, led by the brash American DJ "The Count" (Philip Seymour Hoffman), quickly accept Carl as one of their own.
At the start of the film Doctor Dave (Nick Frost) and Carl conspire to have Carl lose his virginity. Dave has his girlfriend get naked and lie in bed for him while he turns off the lights and goes into the bathroom, where has has Carl hidden. Carl then gets naked and walks into the dark room in an attempt to deceive Dave's girlfriend. However she turns the lights on and sees who was really about to have sex with her, and Carl gets embarrassed and leaves.
In London, government minister Sir Alistair Dormandy (Kenneth Branagh) resolves to shut down pirate radio stations due to their commercialism and low morals, instructing his subordinate Twatt (Jack Davenport) to find legal loopholes that will serve this end. They attempt to cut off the stations' revenue by prohibiting British businesses from advertising on unlicensed radio stations. Quentin counters this by bringing massively popular DJ Gavin Kavanagh (Rhys Ifans) out of retirement and onto Radio Rock, enticing his advertisers to work around the law by paying their bills from abroad. Gavin's popularity creates a rivalry between himself and The Count, who was initially brought to Radio Rock as Gavin's replacement.
On his eighteenth birthday Carl is introduced to Quentin's niece Marianne (Talulah Riley) and falls instantly in love with her, but is heartbroken when she is seduced by Doctor Dave (Nick Frost). Carl's roommate "Thick" Kevin (Tom Brooke) observes that the sex, drug, and alcohol-filled atmosphere of Radio Rock is clearly no place for Carl to get on the straight-and-narrow. He theorises that the real reason Carl's mother sent him there is that his fatherâwhom Carl has never metâis someone on the ship, with Quentin being the likeliest suspect.
DJ "Simple" Simon Swafford (Chris O'Dowd) marries Elenore (January Jones) in an onboard ceremony, but soon learns that she only married him as a means to live on the ship and be with Gavin, with whom she is infatuated. The Count challenges Gavin to a game of chicken in defence of Simon's honour: The two climb one of the ship's radio masts in a clash of egos, reconciling after they are both injured by jumping into the ocean below. When Carl's mother Charlotte (Emma Thompson) visits for Christmas, she denies his suspicion that Quentin is his father. As she departs, Carl passes on a cryptic message from reclusive late-night DJ "Smooth" Bob Silver (Ralph Brown), leading to the unexpected revelation that Bob is actually his father. Soon afterwards, Marianne returns to the ship and apologises to Carl for sleeping with Doctor Dave. She and Carl have sex that night. The following morning, The Count and the rest of the DJ's announce the news of the event to millions of cheering fans all over Britain.
Meanwhile, Dormandy's mission to ban pirate radio advances when Twatt comes across news of a fishing boat whose distress call was blocked by Radio Rock's powerful signal. Twatt proposes the creation of the Marine Offences Act, which will make pirate radio stations illegal on the grounds that they endanger other vessels. Despite public opinion being heavily in support of the pirate stations, the Act passes unanimously through Parliament and takes effect at midnight on 1 January 1967. The Radio Rock crew choose to defy the law and continue to broadcast, firing up the ship's engine so that they may avoid arrest by relocating. The aging vessel cannot take the strain, causing the engine to explode and the ship to start sinking. The DJs broadcast their position in hope of aid, and Twatt appeals to Dormandy to send rescue boats, but Dormandy refuses. Carl rescues the oblivious Bob from his cabin, while The Count vows to continue broadcasting as long as possible.
With the lifeboats inoperable, all gather on the prow as the ship begins to go down. They are rescued by dozens of fans, who heard about their broadcast predicament, and have motored out in a fleet of small boats to save them; Carl himself is rescued by Marianne. The Radio Rock ship disappears beneath the sea, with The Count emerging from the sinking vessel at the last moment. Though pirate radio in Britain comes to an end, the music lives on, with rock and pop becoming increasingly popular in subsequent decades, broadcast over hundreds of 'legal' stations around the world.
Who does Marianne rescue?
Carl