Ian (Ewan McGregor) and Terry (Colin Farrell) are brother's who are both down on their luck financially. Ian falls in love with a a beautiful actress and aspires to leave his father's restaurant business and invest in some hotel properties in California, but he doesn't have the money do it. Terry is a ne'er do well who drinks and gambles, losing $90,000 one night in a game of poker. Both brothers approach their wealthy Uncle Howard (Tom Wilkinson) and ask for money. Howard will help them but he has a favor to ask - a former business partner, Martin Burns, is set to come out with damaging information on Howard which could result in Howard going to prison. Howard asks the boys to kill Martin.
They mull over this proposition and finally decide to do it, knowing that the money they get for this favor could help them start the new lives they have been dreaming of. Ian and Terry succeed in killing Martin one night and seem to get away with it. However, guilt soon begins to drive Terry insane - he starts drinking heavily, taking pills and eventually tells Ian that he is going to turn himself into the police. Uncle Howard and Ian agree that Terry must not be allowed to go to the police so Ian arranges to take his brother out on their boat, Cassandra's Dream, for a cruise where he plans to drug Terry and push him overboard, making it look like an accident. However, Ian loses his nerve at the last minute and instead starts a fight with Terry, telling him he'll ruin everyone's lives if he goes to the police. Terry hits Ian, who crashes into a table and is killed instantly.
The final scene shows the police investigating the accident on the boat, explaining that one brother (Ian) was probably accidentally killed and the other brother (Terry) committed suicide.