Do Not Disturb opens with a cut that explains the marriage at its centre before anyone speaks. Bird’s-eye shots of sunny Miami, edited to an upbeat tempo, are intercut with a slow, quiet scene: Jack asleep in the back seat of a car while Chloe looks at him, despondent. Rue Morgue reads that juxtaposition as the film’s thesis, that nothing, not even paradise, can fix what is broken between these two. The reading holds. John Ainslie, who wrote and directed, spends his first minutes telling us that the honeymoon is a formality, and the rest of the film never takes it back.

Chloe, played by Kimberly Laferriere, and Jack, played by Rogan Christopher, have come to Miami to rescue a relationship that is already failing. Mark Reviews Movies puts it bluntly: if this couple was ever happy together, the honeymoon phase ended well before the trip. The same review calls the tension between them palpable at every moment, and that tension is the film’s real subject. Rue Morgue frames the whole story as a breakup story, and its headline goes further, calling it a cannibalistic one on bath salts. The line is funny, and it is also a fair measure of how far the film travels.

Most of the story happens in one cramped room. Mark Reviews Movies describes a hotel room without a view; Rue Morgue calls it a dingy motel room. Both agree that it is small, unlovely and impossible to share without friction. The lack of a view turns into a fight of its own. Chloe had hoped for an ocean view, since the hotel stands right by the beach, and Jack refused to pay extra for something he considered insignificant, because the beach is right there. Mark Reviews Movies insists that this is not really about money. Chloe has a steady job, handles most or all of the couple’s expenses and has been generally fine with that. Now she wants to be married, to start a family and to see Jack take some initiative, so that she knows she can count on him when it matters. A quarrel over a window becomes a quarrel about whether he can be relied on at all.

The portraits are not flattering, and the film does not pretend otherwise. Rue Morgue sums up Jack as possessive, apathetic and non-supportive, and Chloe as manipulative, resentful and paranoid. They enjoy pushing each other’s buttons, and the audience learns their triggers fast. Drugs are one of those triggers. Jack wants to take them, even though he promised his wife he would quit. Chloe does not want to, because she knows what her husband becomes under the influence. Mark Reviews Movies notes that even the decision to partake is a point of contention. So when the two decide to drink heavily and take assorted drugs in order to relax, and perhaps to find some of the old connection in the stupor, it looks less like a reconciliation than a ceasefire with bad odds.

The turn comes amid Miami’s neon and sunshine, where the couple decide that a peyote experience will strengthen their marriage. They do not know that what they have been given is a rare and powerful strand, and it sends them on a psychedelic exploration of love, lust and carnal desire. Rue Morgue is more specific about the price: the drugs, found on the beach, bring the pair closer together at the cost of an insatiable appetite for other human beings and for each other. By then, that review argues, the audience is in the same boat as Chloe and Jack. We want them to try literally anything that might mend the marriage, for their sake and for ours. The quarrelling has worn us down so thoroughly that we welcome the very thing that will wreck them, and that is where the film gets its nerve.

Mark Reviews Movies locates the surprise of Ainslie’s screenplay in two places. One is the way it keeps finding new ways to escalate the tension, making what seemed the height of discomfort more uneasy still. The other is the specific way the story turns into an unsettling horror story that, in a strange way, sees the couple working together at their best even as they indulge in some of the worst behaviour imaginable. This reversal is the most interesting thing in the film. A marriage that could not agree on a view of the sea finally cooperates once appetite takes over. Teamwork arrives at last, and it costs a great deal.

Rue Morgue places the film among drug-fuelled horror such as Climax, Swallowed and Placebo, which prey on our fears of losing control and descending into madness. The theme fits. But Ainslie’s picture is built differently from those titles: two people, one room for the better part of the running time, and a marriage standing where a cult or a party might be. The comparison tells you what kind of fear is on the menu, and the film still runs on its own engine.

The performances matter more than usual, because there is almost nothing else on screen. Rue Morgue is frank about Christopher: his Jack is often stiff and awkward. But the actor comes to life in the scenes where Jack is tripping, where his physical comedy and confused affect are, in that review’s words, believable and hilarious. An actor who loosens up exactly when his character loses control suits a film about losing control. Laferriere has the harder assignment, because Chloe is described as resentful and paranoid and the film still needs the audience to care whether she gets out of her own marriage in one piece. Janet Porter, Christian McKenna, Patrick McNeil and Rupinder Nagra fill out the credits, but the reviews quoted here keep their attention on the couple, and so does the film.

The weakness is the one Rue Morgue names. The film is almost too simple in its handling of the characters as individuals. We never learn what they sacrificed to keep the relationship going, what they would lose if they split, or how their lives might improve if they left each other. With those questions unanswered, the stakes drain away, and the audience is left apathetic about whether the marriage survives. Mark Reviews Movies gives a partial answer, and it is a good one: Chloe’s job, her expenses and her wish for a family give her a grounded, understandable history. Jack gets far less. He is defined mostly by the promise he breaks. The imbalance leaves one half of the couple a person and the other half a problem, and the horror lands harder on the person than on the problem.

Do Not Disturb is at its best when it keeps its attention on two people who cannot stand each other and then hands them an appetite neither can control. The cramped room, the argument about the view and the first sullen drinks do the real work. The cannibalism arrives as consequence rather than shock. What holds the film back is the thinness of what the pair stand to lose. It is a modest horror film built around a sharp, unkind observation about how a marriage can fail and still function as a team, and that observation is worth the running time.