"The Changeling" is the most entertaining -- indeed, irresistible -- bad movie since "The Concorde -- Airport '79."

A supernatural thriller about the strange affinities between George C. Scott, a bereaved composer, and the cavernous haunted mansion he's rented through real estate agent Trish Van Devere, "The Changeling" is fabricated around a plot that occasionally leaves you in stitches.

The premise isn't outrageous: The house simply harbors a restless, tragic spirit that perceives its new tenant as a sympathetic listener. Rationalizing this communication provokes moments that are sometimes more sidesplitting than thrilling, but the movie's howlers have a funny way of making the spookly atmosphere even more ingratiating.

There are a number of killing lines, beginning with Van Devere's bland observation that this monster house "needs a lot of work." Titillating illustrative details are even more plentiful: Nothing surpasses the fantastic reappearance of a playful rubber ball that Scott has thrown in the river, although Van Devere being pursued by a runaway wheelchair will also be long remembered.

Happily, director Peter Medak is aware of the fundamental absurdity of his ghost story. In fact, he's taken considerable care to compensate with virtuoso displays of scenic and atmospheric suggestiveness. "The Changeling" has a stylistic gusto and polish that were conspicuously missing from "The Fog" and "The Amityville Horror."

Medak's key pictorial collaborators would appear to be conematographer John Coquillon and production designer Trevor Williams. There are at least two awesomely eerie compositions: a majestic overhead shot of a car driving up to the mansion on a tree-lined private road, and a climactic slow-motion shot of a chandelier swinging across up a burning staircase. The somber elegance of the production as a whole provides ironic enhancement to the silliness.

The mystery begins with a prologue in which Scott's wife and daughter are killed in an automobile accident. Hoping to escape tragic memories, Scott leaves New York to accept a teaching post at a university in the Pacific Northwest.His desire for a secluded location where he can compose without disturbing the neighbors is more than satisfied by Van Devere's selection of the Old Carmichael Place, curiously uninhabited for half a century or so.

If ever a residence screamed "HAUNTED HOUSE!" at movie characters, the Carmichael joint is it, but that's part of its cinematic charm.

Williams supervised the construction of the ominous rooms and staircases at a studio in Vancouver, B. C., the principal location. For all practical purposes, his wonderful settings become the "co-star" of the film. Scott's most effective interacting is done with the house rather than any member of the cast.

It's not that Scott fails to relate to his fellow actors. In fact, he and Van Devere threaten to slip into a tongue in-cheek familiarity that puts their cliched dialogue at a terrible disadvantage. It's simply more novel and amusing to watch Scott communicating with the house -- lifting an eyebrow in thoughful apprehension here, throwing a penetrating glare there, recoiling in shock eleswhere.

Medak's indirect methods of depicting supernatural terror produce several palpable shocks and one fascinating sustained sequence -- a seance in which the usual hocus-pocus is cleverly sublimated by showing the medium -- played by Helen Burns -- transmitting messages from the beyond in a frenzy of automatic writing. Her scribbles are subsequently verified when Scott plays back a tape of the seance and suddenly hears the same words being whispered by a presence previously unheard as well as unseen.

Medak's one arguable lapse occurs when he recalls the house's curse, a case of infanticide. Medak appears to linger over this crime. On every other occasion the eruption of violence is explosively effective and swiftly illustrated, although relatively free of gore and sadistic contemplation.

Melvyn Douglas adding another character to his recent gallery of elder statesmen, plays a senator implicated in the house's dark secrets. How you-see-them, now-you-don't guest apearances are made by Jean Marsh, Barry Morse (looking crazed as a psychie researcher) and John Colicos.

Particularly good in crucial small roles are Burns as the medium, Eric Christmas as her solicitous husband and Ruth Springford as a colleague of Van Devere's obviously up to no good ("It doesn't want people!" she seethes, upon learning that the mansion has been rented). Madeleine Thornton-Sherwood, the former Madeleine Sherwood, turns up in the superfluous role of Van Devere's mother, but her panic on seance almost justifies the superfluorsness.

The score by Rick Wilkins seems effective in the romantic-ominous corridor that John Carpenter kept pacing so monotonously in both "Halloween" and "The Fog." The producers of "The Changeling," Joel Michaels and Garin Drabinsky, were also behind Darryl Duke's "The Silent Partner," suggesting that they may have a flair for spotting effective genre material and attracting the right people to stylize it.

Medak, a Hungarian who emigrated to England after the 1956 uprising and began directing in 1967 with "Negatives," should finally have a major commerial breakthrough. He has said he hoped to recall the pleasures of haunted house thrillers like Lewis Allen's "The Uninvited" and Robert Wise's "The Haunting." He's proved that he's no slough at this tradition.