The film opens with a cashier being charmed by Lehiff (Colin Farrell) who after flirting with the girl smashes her in the face and steals from the till. It quickly moves to John (Cillian Murphy) and Deirdre (Kelly Macdonald) who are a recently separated young couple. The film will revolve around their extended friends.
It is quickly revealed that Lehiff is a petty criminal always involved in trouble. Lehiff's nemesis, Garda Detective Jerry Lynch (Colm Meaney) who presents himself as a saviour whose main mission is to fight the "scumbags" on Dublin's streets. He enlists the help of Ben Campion (Tomás Ó Súilleabháin), an ambitious film-maker and the bane of his "go-softer" boss, who considers Lynch too nasty a subject to be shown on a mainstream “docusoap” series on Irish television.
Next up is Mick (Brian F. O'Byrne) a Dublin bus driver. While on his route Sally (Shirley Henderson) boards and is shown to be deeply insecure about her looks. She asks Mick about some hair on her lip and he mocks her playfully. As the bus journey continues a young boy called Philip is shown throwing a rock at his bus resulting in a bad crash that Ben winds up shooting the aftermath of. Ben is told to focus his attention on Sally, Deirdre's sister, who helped the passengers after the double-decker bus crashed. She grows bitter when Deirdre flaunts her new boyfriend, Sam (Michael McElhatton), a middle-aged bank manager who has left his wife of 14 years, Noeleen, leaving her to question her own self-worth as a woman and wife.
John is utterly lost without Deirdre and is determined to win her back. Mick having become suspended from his job and low on funds come up with a scheme involving Lehiff and Sam. They kidnap Sam and force him to go to his bank to get money for a ransom. Just as the plan seems to be working out everything goes wrong as Sam, is assaulted by his enraged wife Noeleen on the street and Gardai are forced to intervene. Mick and John flee the scene without their money.
Mick, later loses his job after he is wrongfully blamed for the bus crash and he becomes obsessed with taking revenge on the kid behind it called Philip. After chasing the kid down in his car he loses control and is left balancing over the canal. Philip is sitting on the bonnet and jumps off letting the car drop into the canal.
Detective Lynch chases down and corners Lehiff in an open field, and decides to take him on while Ben films everything. Unfortunately he miscalculates and Lehiff gets the upper hand and threatens to kill him. Ben snatches at the gun and shoots Lehiff and Lynch covers it all up.
As the credits roll, Noeleen and Sam are shown to have reunited in their house watching television. She is sitting purposely on the remote control and bullying him into changing the channels by hand.Colin Farrell as Lehiff, a petty criminal with a penchant for getting into trouble with the law
Kelly Macdonald as Deirdre
Cillian Murphy as John, Deirdre's former boyfriend
Colm Meaney as Detective Jerry Lynch
Shirley Henderson as Sally
David Wilmot as Oscar
Deirdre O'Kane as Noeleen
Michael McElhatton as Sam, a middle-aged bank manager
Tomás Ó Súilleabháin as Ben Campion, an ambitious film-maker
Brian F. O'Byrne as Mick, the bus driver
Ger Ryan as Maura, the mother
Jane Brennan as Mrs. Rooney
The movie earned €2.5 million at the Irish box office, briefly becoming the most successful independent Irish film.
The film was well received by critics. Rotten Tomatoes gave the film an aggregate rating of 73% based on 93 reviews, with the critical consensus describing the film as "An edgy and energetic ensemble story".
Noted critics Roger Ebert and Richard Roeper gave the film good reviews. Roeper described it as "a likable film about nasty people".
Patrick Condren became the first Irishman to be nominated at the Taurus World Stunt Awards following his work in the film. In the biggest single stunt ever filmed in Ireland, a double-decker bus was flipped 20 metres into the air.
Intermission took $5 million to make, but it didn't do well in the box office, grossing just $4.8 million worldwide.