A classic sea yarn that mixes haunted house tropes with a nautical narrative, “Mary” might not venture into uncharted waters, but it consistently stays afloat. The story of a family sailing into the Caribbean for a fresh start and a little adventure, the film hits most of its marks throughout its brisk 78-minute (pre-credits) runtime. A bit overwritten at times and capped off by an ending that feels a touch rushed, “Mary” cruises all the same: making good use of a balanced set of performances and a steady hand behind the camera.

“Mary” is broken into two alternating timelines, with family matriarch Sarah (Emily Mortimer) speaking in the present to a police detective about the disappearance of her husband, and a flashback based on Sarah’s testimony that details the events that led up to this. In the flashback, Sarah explains that her spouse, David (Gary Oldman), purchased an abandoned sailboat a few weeks ago to start a new business for the family. A fishing charter captain, David saw the boat as an opportunity to start his own business, and in fixing up the vessel, a way to bring his wife and two daughters closer together.

The flashback portion offers the audience a few subtle clues early on that David and Sarah’s marriage had been suffering, but the new boat and the couple’s determination to get through the rough patch seemed to help them through this. The pair set off from southern Florida with their two daughters and two additional crew, laughing off warnings that they’d be passing through part of the Bermuda Triangle on a boat with a questionable ownership history. Indeed, it doesn’t take long before their boat, named “Mary,” begins to have an odd effect on everyone aboard.

Director Michael Goi does a fine job building tension during these early portions of the film, establishing enough bad juju in scattered pockets throughout the first 30 minutes to allow for the organic germination of terror in the back half. Much like a haunted house, the boat has a seductive nature that draws its victims in, using the desolation of the open ocean like the blizzard or storm that keeps a house’s occupants trapped within. It’s doubly effective in an ocean setting, too, as the lapping of water against the hull or the shimmering of moonlight off a wave is enough to make sane-minded people imagine things, let alone those trapped on a haunted vessel.

If there’s anything that pierces this tension bubble, it is the structure of the picture itself, which is anchored to this dual-timeline device. Sarah’s telling of the story, and the revelation in the first few minutes that her daughters are alive and well, robs “Mary” of much of the tension found in the flashback portion, as the audience knows that no matter how bad things get at sea, these characters will survive. Making matters worse, any time a situation gets especially tense on the boat during Sarah’s flashback, there’s the tension release valve of smash-cuts to the police interrogation to draw the audience out of the moment.

This dual timeline structure does have a payoff near the end to reward the conceit, yet it would have worked just as well in a linear set-up that kept the well-earned tension of the flashback portion intact. Which is to say that all of this works just fine, but with a tighter script, it could have really stuck its landing.

Oldman, who’s capable of chewing the sails, rigging, and hull of any scene he’s in, bucks expectations in “Mary” to play things steady and evenly. His character, somewhat hobbled by hurdles thrown in front of him in both his personal and professional life, is stoic and easily assuaged all at once. Watching “Mary,” one can see the actor making a deliberate choice with the performance, and on the whole, it seems like the right one. Mortimer, on the other hand, acts as the lens through which the story is told throughout both timelines and must carry the weight of the narrative’s twists and turns. Her interplay with Oldman comes off as weighty and full of history, and when she has to turn things up during the final act, she never misses a beat.

Top-notch set design and sound mixing help to establish a definite sense of place on the ship, which is vital to the escalating tension that grows more claustrophobic by the minute. And while the structure of the film does hamper the overall effort, the bones of a good horror suspense thriller remain. Gripping, intriguing, and well-paced, “Mary” overcomes most of the issues with its overwritten script to emerge as a serviceable entry in the genre’s canon. Sure, the film lists from time to time, but it always manages to right itself when it matters. [B]