"In Bruges" is a black comedy that enchants us for its first hour or so with its wit and its marvelous premise, which is that two hitmen from London are sent by their boss to hide out in that medieval city until he summons them again; and then in the last forty minutes the film loses its way, and its wit, only to end up as a serious drama.

Brendan Gleeson, as Ken, and Colin Farrell, as Ray, are the two mismatched killers, sent to Bruges to cool out while they wait to hear from Ralph Fiennes, as Harry, their boss. Ken, the sophisticated Londoner, is enchanted by the medieval architecture, the canals, the bricks of the old city, the great church in the central plaza; Ray, the Dubliner, finds it excruciating to be in such a place with nothing exciting to do. But Ray has something on his conscience, which is that while doing Harry's business of murdering a priest during confession, he also managed to kill a child nearby.

In the meantime, though, Ray finds a lovely girl, though it turns out that she and her partner happen to be in the business of robbing tourists, until Ray puts an end to it in an unexpected and very funny scene. Along the way the two hitmen meet Jordan Prentice, who's acting in a film being shot in Bruges, and whom Ray keeps referring to as a midget rather than a dwarf.

Written and directed by Martin McDonagh, whose short film "Six Shooter" won the Academy Award in 2006, the film has the kind of propulsive pacing and great digressions that incorporate the unexpected but delightful moments that are perfect for a black comedy. And then when it came time to resolve the plot McDonagh somehow lost his way, or lost his nerve, or maybe the studio insisted on a conventional ending, but whatever the reason "In Bruges" turns from a black comedy to a kind of black tragedy - something that never should have been allowed to happen.

The acting is superb, with Colin Farrell finally getting the chance to let his inner child out, and Brendan Gleeson having a ball as his older, wiser mentor. Ralph Fiennes as their boss is unfortunately confined to a one-note performance, when in fact he could have taken the film to an unexpected, witty end. Instead he becomes the one who drags the film to a conventional conclusion. "In Bruges" is two-thirds of a great black comedy; maybe that's enough these days.