“Lucky Number Slevin” starts out in fine form. Writer Jason Smilovic blends noir with comedy, crafting an irreverent plot surrounding seven gruesome murders, a racehorse named Seven and a mysterious guy named Slevin. The numbers alone seem to promise a damn good time, but, unfortunately, the film’s luck doesn’t last.

Going for the type of grand “gotcha!” pioneered by Bryan Singer in “The Usual Suspects,” this disappointing gangster comedy just isn’t clever enough to pull it off. Instead “Slevin” sacrifices great characters and a promising plot for a dud of a trick ending.

The film begins with a series of seemingly unconnected events. Mr. Goodkat (Bruce Willis), a cold-hearted contract killer, arrives in New York and offs a guy in the airport, just as Slevin (Josh Hartnett) steps out of the shower to answer a knock at the door — Lindsey (Lucy Liu), from across the hall, has come to borrow a cup of sugar. She is surprised to see Slevin in place of Nick, her neighbor. Slevin claims to be Nick’s friend and the two hit it off. Meanwhile, the film flashes back to the late 70s, telling the story of a likeable American family massacred after a bet on a rigged horse race goes awry.

Back in the present, two goons come in and kidnap Slevin, taking him to The Boss (Morgan Freeman), the leader of a powerful Black mob ring. Mistaking Slevin for Nick, The Boss coolly details his feud with The Rabbi (Ben Kingsley), a rival Jewish crime boss. Ignoring Slevin’s protests, The Boss sends him to kill The Rabbi’s son. Meanwhile, a hard-ass detective, Brikowski (Stanley Tucci) who has been keeping his eye on the “darkies and skullcaps” wants to know who this new guy Slevin is.

The smart plot setup keeps things entertaining. Delicious touches abound: The Rabbi and The Boss live in identical penthouses on opposite sides of the same street, Slevin is dragged around the city in a bath-towel and every scene is wrapped in Smilovic’s snappy, smart dialogue. The film even makes up a disease (Adorexia) that renders Slevin fearless so he can flippantly discourse with each gang boss. And the dichotomy between the Black and Jewish gangs only adds to the fun.

The inspired silliness reaches its pinnacle in the tete-a-tete between Slevin and Lindsey. Not only do Hartnett and Liu have flirtation down to an art, they are masters of comic timing. It is no small testament to their talent that the simple act of eating a peanut butter sandwich becomes one of the funniest bits in the movie. Liu is particularly delightful, and even manages to charm the frequently-rigid Hartnett into loosening up. There is genuine pleasure to be had from the comfortable ease with which lines are tossed back and forth. And it doesn’t hurt that both actors are so easy on the eyes.

Unfortunately, Smilovic takes all this stored-up capital — the great performances, dialogue, preliminary plotting — and wastes it. Shedding its humor, “Slevin” gets mired in needless exposition. In fact, the film’s entire second half is devoted to explaining its first half. This insult to the audience’s intelligence is coupled with a marked decrease in style. As McGuigan doggedly sprints toward the finish, the colorful, quirky cinematography and smart writing that graced the first hour fades out. Instead, “Slevin” betrays its playful characters with tired nods to Tarentino flicks and a silly Shakespearean ending which is grossly out of place. Completing the catastrophe, the film’s most successful character, Lindsey, makes only one second-act appearance.

The only scene worth saving is a testament to the talent wasted on this doomed enterprise. Between the wink-wink, nudge-nudge “Pulp Fiction” allusions, two great actors — Kingsley and Freeman — quietly act out a reunion scene full of remorse, pathos and wit. Watching these two masters rise above Smilovic’s banal plotting only makes the surrounding fluff all the more reprehensible.

Kingsley and Freeman’s sad, somber performances belong to a different film — the film “Slevin” could have been.