When taking on a misguided or unflattering part, actors are sometimes said to have committed career suicide. Knight and Day, an action comedy that's nonsensical from the title down, must be one of the first instances of two actors simultaneously saying: "Goodbye, cruel world." Tom Cruise and Cameron Diaz are the signatories of the suicide pact in question. If they come back from this one, Lazarus will have nothing on them.

Diaz plays June, whose dogged effervescence suggests she is about to say or do something perfectly charming, in the same way that the accordion on the soundtrack promises a certain European quirkiness or sophistication. June is a good match, then, for Roy (Cruise), a fellow passenger with whom she flirts openly on a flight out of Wichita, Kansas. Roy's knowing demeanour also indicates that a witty line is just around the corner. And just around the corner is where it stays.

What June doesn't realise is that Roy is a secret agent and wrongly suspected turncoat. While she's in the bathroom on the plane, there's a shoot-out and he despatches the other passengers, who've been sent to bump him off. The writer Patrick O'Neill has come up with the dotty idea of having June discover the carnage only once the plane banks and the corpses roll sideways in their seats. James Mangold, previously director of such gigglefests as Walk the Line and Girl, Interrupted, fudges the rhythm of this promising gag, which remains only theoretically amusing.

It does, however, begin a running joke in which June proves oblivious to the risks around her. Her already limited powers of perception can hardly be sharpened by Roy's special fondness for drugging her in order to transport her from one destination to another as he goes on the run and attempts to clear his name. That he undresses June while she's insensible is presented as evidence of how delightfully incorrigible he is. Each time she regains consciousness in some part of the world, the film moves closer to being the world's first Rohypnol romcom.

On those occasions when she is sentient, June tends to flap hysterically. When required to handle a weapon, she pulls the trigger in panic. ("What else would a woman do when given a gun?" the film seems to be asking.) Not that Roy is any more complex. It's part of Tom Cruise's unique burden that the delivery of a throwaway quip seems to cause him to suffer an emotional hernia; lightweight really shouldn't feel this heavy. It doesn't help that Roy is a man of blasé brutality who thinks nothing of hitting a passing waiter in the face with the butt of a gun, or shooting a fireman in the leg. The 1988 film Midnight Run moved far more successfully between violence and comedy, but then that had two advantages: its action scenes were exciting and its humorous lines were funny.

The overwhelming impression given by O'Neill's script is that of entire scenes in which blank spaces have been left, along with a note to self that reads: "Insert comedic line/ scenario here. See Charade for ideas." Ineptness creeps into every department. High-calibre performers such as Paul Dano, Viola Davis and Peter Sarsgaard appear to sag as it slowly dawns on them that they won't emerge from this one with honour. Simple moments of jeopardy fall flat. When June's face is held close to a deep-fat fryer during a fight, you don't gasp; you just think, "Oh, that beautiful complexion!"

The only possible fascination that Knight and Day provides lies in its attempts to tip us off about the demise of Tom Cruise's career. Much like hostages raising the alarm when the kidnapper's back is turned, the movie keeps issuing coded reports of its leading man's death, while appearing to flatter him with shots of Diaz sighing over his buff body. In one of his first scenes, Roy plays an arcade game that informs him: "You're Dead." Later in the film, he has cause to pose as a corpse. We also discover that his parents believe he was killed in combat. For anyone who knows their Beatles lore, this can only feel like the "Paul Is Dead" myth all over again. In this case, the evidence seems incontrovertible.