A couple experiences love and loss and loss and loss and loss in “The Light Between Oceans,” Derek Cianfrance’s new period drama set in post-World War I Australia.

Tom (Michael Fassbender), a veteran of the Western Front, returns to the west coast of Australia, wracked with survivor’s guilt. He takes a job as a lighthouse keeper on a desolate island, a long boat ride from the nearest town, where he meets local girl Isabel (Alicia Vikander). A cliffside picnic backdropped by a breathtaking sunset and a few love letters later, and the two are married and living together on the island.

All’s not well with the lovely couple, however. After a few unsuccessful attempts to bring a child into the world, Isabel has taken a thousand yard stare not dissimilar from Tom’s, at least until a dinghy with a dead man and a live baby wash ashore. Tom insists they report it, but Isabel sees this as their last chance at parenthood. Things get complicated when Tom finds another local woman (Rachel Weisz) mourning at a tombstone dedicated to the memory of her husband and newborn, lost at sea.

Cianfrance made his name in 2010 with “Blue Valentine,” a heartbreaking film about a couple falling into and out of love, its devastating quality borne from the authenticity of both the romance and the resentment. Cianfrance is still one of the premier romantic storytellers in Hollywood, but this film’s second and third acts don’t hold up as well as the lovely first. Adapted from the M.L. Stedman novel of the same name, Cianfrance shows that conventional plots do nothing but hamper his vision.

Featuring a plot with murder charges, xenophobia and a custody battle, the danger in “The Light Between Oceans” somehow seems less urgent, less dire than the domestic drama of “Blue Valentine.” Fassbender and Oscar-winners Vikander and Weisz do the syrupy script and its voiceover-narrated letters more than justice, but the 132-minute running time had me wondering when King Solomon would come in and halve the kid to wrap up the story, and that was before the maize content reached dangerous levels in an unnecessary epilogue.

In addition to the surehanded direction and great performances from the leads, cinematographer Sean Bobbit loads the film with gorgeous shots of the island and the surrounding sea, and composer Oscar winner Alexandre Desplat contributes yet another excellent score.

“The Light Between Oceans” begins with loads of promise before it is sandbagged by predictable cliches. While it’s exciting when indie darlings like Cianfrance are given bigger budgets to play with, it’s disappointing when their works turn conventional. Perhaps the movie about an island-bound couple encountering a sea baby would be much more interesting if the baby had caught a different tide.