"Knight and Day" feels like an event film if only because of its pairing of two major modern-day movie stars, a Hollywood hallmark that no longer happens as frequently as one might expect. After all, why pay the big bucks for a couple long-time A-listers who command millions of dollars per picture when you can spend the money on one and then get a cheaper actor to play the leading co-star? For a time—maybe the first forty-five minutes or so—"Knight and Day" lives up to and even surpasses one's expectations. An action-comedy-romance hybrid that keeps the details of its plot close to the vest for a long time, one is immediately intrigued and delighted by the snappy writing by Patrick O'Neill, the smooth direction by James Mangold (2007's "3:10 to Yuma"), and the instant blaze-worthy chemistry between Tom Cruise (2008's "Valkyrie") and Cameron Diaz (2009's "The Box"). It's fun not knowing where things are headed, and what the immediate motives are of its characters. The viewer happily settles in for surprises and revelations to come as the actors' charisma leads the way. Sadly, when they finally do arrive, they are so hackneyed and underwhelming that one's excitement instantaneously deflates like a blown-up balloon that hasn't been knotted properly.

Roy Miller (Tom Cruise) has his eye on June Havens (Cameron Diaz) the moment he spots her at the airport, orchestrating two separate episodes where they bump into each other before getting on the same plane. With few other people onboard, they strike up a conversation about their respective bucket lists—the things they've always dreamt of doing, but never have. There is an undeniable attraction between them. As June visits the restroom, planning her next move, Roy is besieged by the other passengers, flight attendants and pilots, killing the lot of them. With Roy now at the helm, they crash-land in a cornfield. June is understandably freaked out by the turn of events and wakes up the next morning in her own bed in Boston. This is not the last June will see of Roy, and soon both of them are being chased down by crooked men looking like federal agents. Who, exactly, is Roy? What is it that he is trying to protect and these other guys want? Why is June's life suddenly in danger, too?

"Knight and Day" answers these questions all in due time, and it is at this midway point that the film falls apart and becomes a hodgepodge of ridiculous ideas, extraneous globetrotting, and sloppy storytelling. It would be improper to give away the movie's secrets, but it is probably best to set your prospects very low. With the details of the plot dinky and ham-fisted and the safety of Roy and June never in doubt, interest flags. Characters jump from Boston, to New York, to Austria, to Germany, back to Boston, off to Spain, and then to Washington, DC, without any sense made of how they have gotten there within a single film cut. There is no detectable point for most of these locations save for the filmmakers' desire to shoot in exotic places. Meanwhile, the kindred connection sparked in the early scenes between Roy and June lose steam as the characters take a backseat to the lame machinations of the premise. Grindingly conventional yet not without some grandly loopy, even bizarre, occurrences, one questions at times whether the film is attempting to be avant garde. It's not, though. It's just pieced together more incoherently than usual.

What a shame it is that the picture goes downhill because the first act and a half are really quite impressive. The long opening set-piece set at the airport and on the plane is dazzling—enthralling, genuinely romantic, funny, mysterious, and finally thrilling. An intricate, stunt-heavy car chase follows soon after and is masterful, shot and edited properly for maximum tautness and free of the recent trend to shake the camera erratically to emulate chaos. Tom Cruise and Cameron Diaz are in total control of the screen, reminiscent in performance and sex appeal to that of the golden age of cinema. They are a treat to watch here, clearly having fun and not taking things too seriously. Supporting players, like Peter Sarsgaard (2009's "Orphan"), Viola Davis (2008's "Doubt"), and Maggie Grace (2009's "Taken"), are wasted beyond comprehension in nothing parts. The film's second half loses steam not only in its disappointing plot, but also in keeping Roy and June apart too much. More concentration on them and less on what amounts to "Mission: Impossible 4" would have improved things considerably.

"Knight and Day"—a title that makes only partial sense—is a movie of two parts. The first half is splendid, raising hopes sky-high about what is to come. The second half—what does come next—is a real bummer, a rote, been-there-done-that escapade with middling enthusiasm and narrative imbalance. Attempts at humor aren't as funny. The action becomes more perfunctory. The romance basically takes a vacation until the end, and by then, oddly enough, isn't nearly as powerful as it was in the early scenes. There's a superior motion picture lurking on the fringes of the finished film; it's too bad only glimpses of the magic that could have been ever rise to the forefront.