There’s something missing from Adrian Lyne’s “Deep Water,” and it’s not just the body of Martin McRae, the last unfortunate rival to get a little too friendly with Vic Van Allen’s wife. Vic (Ben Affleck) and Melinda (Ana de Armas) have an open marriage, but her … distractions have a habit of disappearing, and so do pretty much all ties to recognizable human behavior in the “Fatal Attraction” director’s unexpectedly coolheaded adaptation of the 1957 Patricia Highsmith novel for Hulu. This erotic thriller is still sexy and plenty entertaining, mind you, but it’s just not very useful insofar as what it says about real relationships.

Late last century, Lyne had a long, successful run of portraying complex sexual dynamics through grown-up eyes, but it’s been 20 years since “Unfaithful” — he spent two decades fighting to get this film off the ground — and the now-octogenarian helmer’s influence on subsequent sizzlers has undermined his own capacity to shock. Films like “Gone Girl” and “Fifty Shades of Grey” are nothing if not knockoffs of the classic Lyne aesthetic (which treats sex more seriously than its softcore competition), pushing the envelope farther than the director is willing to go with this particular project.

In terms of material, Lyne’s sensibility would seem an ideal fit with Highsmith’s, given their shared preoccupation with jealousy and illicit desire. But it turns out the filmmaker lacks the “Talented Mr. Ripley” writer’s grounded sense of psychology, putting his emphasis instead on suspense — well, that and snails, which occupy a surprising amount of the movie’s attention (but more on that in a minute). Elegant as ever — to a fault — plot-centric Lyne seems more concerned with how things happen than why they do.

Much of this could have been solved rather simply, by including a conversation — or better yet, an argument — between Vic and Melinda in which the couple hash out the rules of their arrangement. They have what’s sometimes referred to as an “understanding.” The problem is, we don’t understand it. As Vic, Ben Affleck looks grizzled and angry for most of the film, glowering at Melinda from across the room at dinner parties as she brazenly flirts with other men. Most husbands would probably have a similar reaction. But most husbands don’t give their wives permission to consort with whomever they please, so long as they agree not to tear the family apart — which happens to be the deal in “Deep Water.”

Vic retired early, comfortably rich, and now serves as a house husband, taking care of their daughter (Grace Jenkins) while Melinda amuses herself on the town. Their evenings are a succession of parties at friends’ houses, at which she inevitably drinks too much and crosses the line. But where is the line? And what is Vic thinking when he catches Melinda making out with handsome idiot Joel (Brendan C. Miller) at one of these soirées?

He stares down from an upper window, catching her eye, and in this exchange, we are supposed to conclude … what? That he’s OK with it? That seeing her with another man turns him on? That Melinda is daring him to react? Maybe even all of the above. Affleck’s expression is unnervingly inscrutable, which could be the right answer in a certain context: People tend to be relatively poker-faced in real life. We could certainly debate whether it’s good acting or bad to telegraph a character’s internal reactions, and yet, in this context, audiences need some kind of clue to know how to read their relationship, and Affleck withholds that.

An early scene, which shows Vic and Melinda retiring to separate corners of the house, suggests the emotional distance that exists between the couple. After the party, Melinda denies him sex and sends him out of the room. But later, after Vic surprises her by dancing with another woman, it sparks a passionate lovemaking session. There’s an enticing puzzle aspect in trying to untangle the codes of their relationship. The trouble is, they’re not consistent, and what he says — to her, or to his friends (Dash Mihok and Lil Rel Howery, always good for a laugh), isn’t necessarily reflective of what he feels. Vic makes lofty claims of accepting her unconditionally — which sets up the movie’s unconvincing ending, different from the book’s (or that of “Eaux profondes,” the French adaptation from 1981, which starred Jean-Louis Trintignant and Isabelle Huppert). But unlike most functional open relationships, he can’t subsume his envy for the sake of her happiness.

Vic goes from cuckold to killer over the course of the film, and it’s not at all clear what flips the switch. In a private moment with Joel, Vic makes what he later describes as a joke, claiming to be responsible for Martin McRae’s disappearance. It’s his way of threatening this impertinent stud, of letting him know he’s not as “cool” with his wife’s playthings as she must claim, and it works. Joel backs off. But the story gets around, and new-to-town neighbor Don Wilson (Tracy Letts) even takes him at his word, going so far as to hire a private eye after another of Melinda’s “friends” turns up dead.

Lyne tantalizes us with the ambiguity of it all for a time, aligning the film’s POV with that of Vic, who retreats to the greenhouse out back to play with his snails whenever his feelings are hurt. Why snails? These hermaphroditic creatures must surely represent some kind of metaphor, too obscure to be easily interpreted. They also serve a more direct, sensual role: In “Gone Girl,” Affleck caught audiences off guard with a glimpse of frontal nudity in the shower. Here, the biggest shock is a scene in which he lets a few of these beloved gastropods slither up his arms.

Vic’s actions get increasingly unbelievable as the movie goes on, but Lyne’s a talented enough director to keep us invested, even in the lunatic last third. If anything, he doesn’t push things far enough. In other words, he’s still great at what he does; he just doesn’t do enough of it.