It’s hard to believe that Derek Cianfrance, whose “Blue Valentine” was so compelling a relationship drama, could have made such a damp emotional squib as “The Light Between Oceans.” This adaptation of M.L. Stedman’s best-selling tearjerker is tasteful, elegant and decorous—but also stiff, overinflated and ponderous. By the end its sappy earnestness has become more than a little risible.

Of course, one must admit that the basic premise of the narrative is far-fetched to begin with. Tom Sherbourne (Michael Fassbender) is an Australian man still traumatized by his experience in the trenches during World War I. Returning to his homeland, he takes a temporary position as the keeper of a lighthouse on remote Janus Island, hoping to use the solitude to overcome his demons. In the town on the mainland, however, he encounters Isabel Graysmark (Alicia Vikander), daughter of the schoolteacher and his wife (Garry McDonald and Jane Menelaus), who lost two brothers in the war and is drawn to him. After what seems a brief exchange of letters, they marry and she moves to the otherwise deserted island with him.

Initially the two are intensely happy, but a problem arises when Isabel suffers two miscarriages and becomes despondent over the prospect of bearing the child she so desperately desires. Fortunately—from her perspective, at least—a dingy shows up in the roiling sea carrying a dead man and a wailing infant girl. Though Tom knows they should report the discovery to the proper authorities, Isabel insists that they raise the child as their own, and he reluctantly agrees, burying the man’s body to ensure secrecy. Over the next four years the girl grows up to be a lovely, garrulous, much-loved tyke the couple names Lucy (Florence Clery).

When the Sherbournes make a trip to town for a celebration, however, Tom learns that their Lucy is actually Grace, the daughter of a still grief-stricken woman named Hannah Roennfeldt (Rachel Weisz), whose husband Frank (Leon Ford) was presumed lost at sea years earlier. (Supposedly he took off in a dingy with his infant daughter after being harassed by locals moved by anti-German sentiment.) Tom feels compelled to inform the woman, anonymously, that her daughter survived and is well, and follows that up by sending Hannah a silver rattle the infant had been clutching. The information leads to a police investigation that leads to his arrest for murder—the assumption is that he killed Frank—and the return of Lucy to Hannah. He is willing stoically to take complete responsibility in order to protect Isabel, though she is of course devastated at having to give up the child. And Lucy/Grace proves unable to deal with a forced change in mothers, either.

This scenario, as implausible as it is, might have a chance of working as a period tearjerker had it been played with full-throated conviction, but Cianfrance seems to have been under the misapprehension that the material is profound drama rather than over-the-top melodrama, and the approach is so solemn and lugubrious that it accentuates the absurdity of the plot without unabashedly embracing it. As a result the film fails to deliver on the promise of leaving misty-eyed viewers reaching for their tissues; when a coda involving grown-up Lucy/Grace (Caren Pistorius) comes around, it might provide narrative closure but is unlikely to elicit tears.

That’s not to say that the actors don’t try; they do. Both Vikander and Weisz emote to the limit; both of them suffer, photogenically, and apply every effort to touch our hearts. Fassbender is a past master of quiet intensity, and simmers profusely here. But all three of them are flummoxed by the deadening directorial pallor, and Cianfrance seems to realize it. He tries to compensate with an avalanche of montages of sea and shore, many of them infected, in Adam Arkapaw’s widescreen cinematography, by sudden bursts of lens flare intended for artistic effect. And when that fails he inserts scene after scene of little Lucy/Grace at play, apparently convinced that Clery is such a charmer that she will automatically makes us care. It’s a manipulative ploy that doesn’t carry the hoped-for punch. The film also suffers from a drippy score by Alexandre Desplat that’s far from his best. It should be added that the supporting cast is okay, though one often feels that they are situated in still compositions rather than asked to bring some zest to their moments on screen.

In sum, “The Light Between Oceans” might aim to generate some emotional heat, but in the end proves a pretty dim weepie—like Nicholas Sparks in period dress.