It’s like Thomas Hardy crafted two thirds of an epic and let Nicholas Sparks finish the final chapters. Derek Cianfrance’s The Light Between Oceans (C), itself an adaptation of M.L. Stedman’s popular post-WWI Australian romance, features Michael Fassbender as a former soldier turned lighthouse keeper and Alicia Vikander as his new bride on an isolating island where intrigue about their offspring becomes the stuff of tempestuous melodrama. The presence of veteran Aussie actors Bryan Brown and Jack Thompson and a few verses of “Waltzing Matilda” are basically the clues you’re down under (thankfully no koalas) long before the narrative seeps below sea level. Fassbender and Vikander are wonderfully charming until their character motivations require them to become a bit stupefying. They commit to their craft well after the story strains credulity. A thankless supporting role does Rachel Weisz no favors. What starts out pulpy, electric and suspenseful simply becomes sudsy, irritating and overlong. Add to this mix an inconsistent narration, an overuse of reading letters and an awkward framing device, and you have the formula for a tepid September studio release.