The film was premiered in secret at Avilés in Spain on June 18, 2007. It was officially premiered at the Venice Film Festival on September 2, 2007 and was already in theaters in Spain by November 3rd, 2007. The film had its North American premiere at the Toronto International Film Festival on September 11, 2007.

The film involves British siblings Terry (Colin Farrell) and Ian (Ewan McGregor), who live in South London, and were raised by a weak father who runs a restaurant, and a strong mother 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 dog that won the race in which Terry earned the money to buy the boat. After a day sailing with their girlfriends and while driving back home in a borrowed Jaguar, Ian crosses an attractive woman on the road (Hayley Atwell), with whom he falls in love. The brothers' financial difficulties, Terry's gambling problems, and Ian's 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. When the two brothers ask for his financial help, he unexpectedly reveals that he's about to go to jail due to certain accusations coming from Martin Burns, an old business partner who plans to testify against him. Howard asks his two nephews to murder Burns.

The two brothers succeed in carrying out the murder uncaught, but Terry's conscience is heavily loaded and he starts to become depressed and suffers from panic attacks. After he confides to his brother that he's thinking about informing the police, Ian plans to poison him during a trip on the boat. In the end, Ian can't bring himself to kill Terry. Thus in a fit of rage, they tussle aboard their small sailboat. In the chaos, Ian is thrown against the hull and killed. Terry, who already had thoughts of suicide from the depression he faced by becoming a murderer, then drowns himself.

This is Allen's third film to be shot in London. It is the first of the three not to star Scarlett Johansson.

According to the Internet Movie Database, the budget is approximately £13 million.

Filming began on July 10, 2006.

This is the first Woody Allen film released with a stereo soundtrack. Allen previously eschewed stereo although often employing Dolby Stereo and Dolby Digital technologies to convey a higher quality mono soundtrack. "Cassandra's Dream" is stereo for its music only.