Though it’s based, as we’re told at the very start, on a true story, “The Vow” comes across as a totally synthetic tearjerker in the Nicholas Sparks mode. It feels like a LifeTime movie pumped into a theatre auditorium, though audiences attuned to such schmaltzy fare will probably embrace it, especially as Valentine’s Day fare.

The crux of the plot is that old standby, amnesia—and even better, partial amnesia. After a prologue showing the cute meeting, courtship and happy marriage of effervescent art student Paige (Rachel McAdams) and studly recording studio owner Leo (Channing Tatum), the story kicks in when their car is rear-ended by a truck on a snowy Chicago street, leaving her in a coma. When she awakens, she doesn’t recognize Leo and has no memory of the previous five years. Her last memory is of being in law school and engaged to her then beau Jeremy (Scott Speedman).

Soon her rich parents Rita (Jessica Lange) and Bill Thornton (Sam Neill), a control-freak judge, show up to tend to her, even though she’d long been estranged from them and they had never even met Leo. After a brief spell back with her husband, now a stranger—which doesn’t turn out well—she decides to retreat to her parents’ suburban home, using as an excuse helping to plan her sister Gwen’s (Jessica McNamee) imminent wedding. But in the event she reconnects with Jeremy and all her old friends from her past life. Bill, sensing an opportunity to regain the daughter he’d lost (and redirect her to the study of law), even suggests divorce to Leo as the natural course of action, since despite the lovesick man’s best efforts, Paige shows no sign of regaining any memory of their life together.

“The Vow” is attractively mounted, with cinematographer Rogier Stoffers giving the images the glossy sheen this sort of material invites. (There are also plenty of loving shots of the Chicago skyscape and some of the city’s landmarks, like the old Music Box Theatre.) But the narrative never develops the sort of emotional punch the makers are aiming for. Part of the problem is script-related. When the writers aren’t engaged in silly bits of business (like having the café where Paige works called the Mnemonic, ha-ha!) it’s confecting ultra-sweet stuff for Leo to court Paige with, both the first time around and the second.

More importantly, though, the screenplay fails to make the lead characters as engaging a pair as they should be, and that has an unfortunate effect on the performances. Leo is just a likable lug, and Tatum plays him in a curiously stolid, low-voltage fashion, too often underplaying the charm (though showing off a good deal of his well-muscled beefcake). Even more troublesome is Paige, whom we should sympathize with but McAdams plays in a mostly stiff, almost shrill pose. Worse still are the supporting parts. Speedman makes an appropriately sleazy foil for Tatum’s Leo, but Neil is an almost comical caricature of the rich, controlling daddy, and Lange swoons and blubbers overmuch as a mother anxious to get her daughter back. (The big revelation about the couple in the last act—which led to Paige’s estrangement from them—is also badly handled from the dramatic perspective.) The smaller roles are at best prosaically handled, further confirmation of director Michael Sucsy’s erratic touch. And the score ascribed to Rachel Portman and Michael Brook is irritatingly obtrusive.

The title of the picture is drawn from the promises that Leo and Paige made to one another at their wedding ceremony, an impromptu (and unauthorized) ceremony that they held before a passel of their artsy-craftsy friends at the Chicago Art Institute, and they come back at the close to emphasize that despite all the difficulties life has thrown at them, the duo will keep the commitment to one another that they represented. It’s a pity the picture doesn’t manage to keep its promise to its target viewership to be a first-rate example of manufactured sentiment. Only the most easily manipulated will have their heartstrings plucked by “The Vow.”