One goes into a film like “Separate Lies” with high hopes. It boasts a script by Julian Fellowes, who wrote “Gosford Park,” and has directed it himself, and an exceptional cast of able British actors. But it doesn’t take long before the movie lets you down. It’s a domestic drama about the destructive effects of infidelity on an upper-class couple, with an almost coincidental subplot about a mysterious hit-and-run that leaves a man dead. But despite the fact that the picture is apparently meant to be emotionally sharp, and even to say something profound about the nature of relationships, guilt and reconciliation, it’s played at so high a pitch that it becomes rather ridiculous, like a spoof of the lesser English soap operas commonplace on PBS. And the acting, especially by the usually reliable Tom Wilkinson as a man whose wife leaves him for an obvious cad (or perhaps in this case we should say “rotter”), goes so over-the-top that at certain points one actually suspects that it is intended to be comical. But unhappily that seems not to have been the case. And as a serious piece “Separate Lies” certainly doesn’t deserve to be taken seriously.

The film begins with road accident, in which a man riding a bicycle is struck down by a speeding Range Rover. Before anything further follows from it, we’re introduced to James and Anne Manning (Wilkinson and Emily Watson), a London corporate attorney and his wife, who have a country house in the rustic district as well as a place in the city. It’s their housekeeper Maggie (Lina Bassett) whose husband was killed in the accident. Also on the scene is a hedonistic, amoral local aristocrat (and peer-in-waiting) named Bill Bule (Rupert Everett), who just happens to drive a Ranger Rover, and toward whom Anne is significantly less cool than her husband (though James is close to the man’s father Lord Rawlson, played by the venerable John Neville). Before long it’s revealed not only that James is being cuckolded, but also that Bill was not the only person involved in the hit-and-run. Shortly thereafter Anne has gone off with Bill and a police inspector (David Harewood) enters the picture as the dogged investigator of the possible case of vehicular manslaughter.

One doesn’t want to go too deeply into the intricacies of the plot, or to reveal the twisty denouement (or more properly series of denouements). But it may be noted that a secondary bit of business involving the torch James’s secretary obviously carries for him goes nowhere, and even the hardiest viewer may blanch when the hit-and-run case remains unresolved, the corpse apparently being of less concern than the amorous trio’s achieving some sense of personal peace. Perhaps these things were more convincing on the printed page–Fellowes’ script is based on a novel, “A Way Through the Woods,” by Nigel Balchin. But on screen it all comes across as slightly absurd. Partially that’s a factor of the story, which never convinces you that it could be about actual human beings, but the cast contribute to the sense of unreality. (If “Separate Lies” does nothing else, it proves decisively that under inauspicious conditions good actors can give terrible performances.) Wilkinson suffers so extravagantly from pangs of remorse and anguish over his wife’s faithlessness that he seems a modern male equivalent of Camille. By comparison Watson is simply bland, but Everett struts about like a refugee from a Noel Coward play, spouting an endless ream of nasty one-liners, and when he falls terminally ill, he still looks positively strapping in a hospital scene in which he’s supposed to be at death’s door. (If British hospital rooms actually look as spartan as the one here, moreover, it’s truly an indictment of the national health service.) And Neville, as Bill’s father, interrupts his almost preternaturally stiff turn with a sequence in which he breaks down over his son’s illness that’s embarrassingly poor. Still, Harewood manages to be even stiffer as the dedicated gumshoe.

In the hands of cinematographer Tony Pierce-Roberts, “Separate Lies” is visually attractive; the exteriors are quite lovely and the interiors either sleekly modern or burnished period. But what happens in front of the settings goes very awry. You’ll find it hard to keep the proverbial stiff upper lip when the overheated melodrama on screen is prompting you to laugh out loud.