At the beginning of “The Vow” — and again at the end, in case you had forgotten — you are told that it was “inspired by true events.” There is always a lot of latitude in such a claim, and no one would accuse this mild, soggy romance of excessive realism. But a cautious desire to remain somewhere in the neighborhood of plausibility also robs the movie, directed by Michael Sucsy from a script with four credited authors, of the opportunity to capitalize on the weirdness and extremity of its premise. When we first meet them, Paige and Leo — the dimply and adorable Rachel McAdams and the decisively chiseled Channing Tatum — are an artsy young married couple enjoying a snowy evening in Chicago. Then a car accident sends Paige through the windshield and into a coma, from which she emerges even more adorable than ever but also with amnesia. She does not recognize Leo and cannot remember anything of their four years together, the highlights of which (cute meeting at the Department of Motor Vehicles; hipster nuptials at the Art Institute; discreet sexy snuggles in their North Side loft) we have already seen in flashbacks. More unsettling is that Paige seems to revert to an earlier version of herself. When she and Leo met, she was estranged from her wealthy suburban parents. She had dropped out of law school to become a sculptor and dumped a skinny fiancé named Jeremy (Scott Speedman). Now that old life threatens to reclaim her. Mom and Dad (Jessica Lange and Sam Neill) are not crazy about Leo, who runs a recording studio and spouts random bits of rock-’n’-roll trivia at the dinner table, and Jeremy still carries a torch. Paige, meanwhile, having lost her touch with clay, lightens her hair and trades in her bohemian garb for preppier attire. “She’s turned into this sweater-set-wearing, mojito-drinking sorority girl,” Leo complains to a friend. “Like a Stepford wife?” “Yeah.” But not really. “The Vow” leaves the creepy implications of Paige’s condition unexplored, which is fine. But the movie’s commitment to the blandest possible presentation of its central problem starts to seem perverse after a while. This could have been a rich, strange melodrama; a psychological thriller; a horror movie; a dark comedy; or any combination of these, and scholarly viewers can relieve the tedium by imagining it remade by more daring filmmakers. Just think of what Alfred Hitchcock or Pedro Almodóvar or Luis Buñuel or John Waters could have done with this material. Never mind. “The Vow” is designed for comfort, not shock. But even by the standards of commercial melodrama it’s a pretty weak brew. Ms. McAdams and Mr. Tatum have both done time in the Nicholas Sparks school of tragico-preposterous inspiration — she in “The Notebook” with Ryan Gosling, he in “Dear John” with Amanda Seyfried — and they have the longing looks, slow teardrops and lip trembles down cold. When they are on the screen together here, there is enough physical charm and emotional warmth to distract from the threadbare setting and the paper-thin plot. But those defects ultimately get in the way of the stars and leave you wondering: Is this a romance about neurological impairment or a neurologically impaired romance? The dialogue — in particular Leo’s half-literate voice-over narration — is atrocious. The secondary characters are less vivid than the extras in an average non-Super Bowl television commercial, with the notable exception of Ms. Lange, whose one longish scene is the film’s only moment of eccentric, unpredictable passion. Otherwise stuff just seems to happen, and not much seems to be at stake, which is odd given that the movie’s ostensible subject is the complex interplay of heart and brain. “The Vow” ends up taking a pro-amnesia line, in two different ways. First by suggesting that memory loss can be a great opportunity to start fresh, re-examine options and resolve long-standing wardrobe issues, and also in its exemplary commitment to its own forgetability. “The Vow” is rated PG-13 (Parents strongly cautioned). A little swearing, a few sexual references and — spoiler alert! — a glimpse of Mr. Tatum’s bare behind. The Vow Opens on Friday nationwide. Directed by Michael Sucsy; written by Abby Kohn, Marc Silverstein and Jason Katims, based on a story by Stuart Sender; director of photography, Rogier Stoffers; edited by Nancy Richardson and Melissa Kent; music by Rachel Portman and Michael Brook; production design by Kalina Ivanov; costumes by Alex Kavanagh; produced by Jonathan Glickman, Paul Taublieb, Gary Barber and Roger Birnbaum; released by Screen Gems. Running time: 1 hour 44 minutes. WITH: Rachel McAdams (Paige), Channing Tatum (Leo), Sam Neill (Bill Thornton), Scott Speedman (Jeremy), Wendy Crewson (Dr. Fishman) and Jessica Lange (Rita Thornton).