British brothers Terry (Farrell) and Ian (McGregor), who live in South London, were raised by a weak father (John Benfield) who runs a restaurant, and a strong mother (Clare Higgins) who taught her sons to look up to their uncle Howard (Tom Wilkinson), a successful businessman.
Both brothers decide to buy a sailboat which they name Cassandra's Dream, after the horse that won the race in which Terry earned the money to buy the boat. Knowing nothing of Greek Mythology, they are unaware of the ominous antecedants of this name - the ancient prophetess Cassandra, whose prophesies of doom went unheeded by those around her.
After a day sailing with their current girlfriends and while driving back home in a borrowed Jaguar XK150, Ian crosses paths with beautiful actress Angela Stark (Hayley Atwell), with whom he falls in love.
The brothers' financial difficulties -- Terry has a gambling problem, while Ian wishes to invest in hotels in California -- lead them to think about asking for Howard's help. Uncle Howard supposedly comes to London to celebrate his sister's birthday, but when the two brothers ask for his financial help, he reveals a more sinister motive. He admits that he is about to go to jail because of accusations coming from Martin Burns, a business partner who plans to testify against him, and asks his two nephews if they can help to get rid of him. After initial reluctance, the two brothers agree.
They make two working zip guns from metal and wood, allowing them to burn them afterwards. In an initial attempt they wait in Burns's home; unexpectedly he is accompanied by a woman. Panicked, they leave and agree to commit the murder the next day.
In the next attempt they succeed in carrying out the murder uncaught, and burn the guns. Ian is content to move on as if nothing happened, but Terry is consumed by guilt and begins abusing alcohol and pills. This frightens his girlfriend (Sally Hawkins), who meets Ian to tell him about the situation, mentioning to him that Terry believes he has killed someone. Terry's behavior begins to get out of control, thus terrifying Ian. After Terry confides to his brother that he's thinking about turning himself in to the police, Ian goes to Howard for advice. Together, they agree there is no other alternative but to get rid of Terry. Ian then plans to poison Terry during a trip on the boat. In the end, however, Ian can't bring himself to kill Terry. In a fit of rage, they tussle aboard the boat. In the chaos, Terry knocks Ian against the hull and kills him. Terry, who was already suicidal, snaps and drowns himself.