Marriage can take on a life of its own, long after the partners have ceased to like each other, and in "Lily in Love," that's a good thing. It's a funny, old-fashioned romance, tender without losing its edge, with a spectacular performance by Christopher Plummer.

Plummer plays Fitzroy Wynn, a vain, orotund actor of the old school, given to much Tamburlaine, more drink, and stentorian, self-serving curtain speeches: "Shakespeare, Marlowe, Dryden -- they're my real cronies," he confides to the audience, with much rolling of r's. His wife, Lily (Maggie Smith), pens plays tailored to his gallant bravado, but she's starting to tire of Fitz both as husband and as protagonist. So when she writes a screenplay, it calls for a hero who's completely different.

Fitz, of course, won't be end-run this way. With a rubber nose, colored contact lenses, a blond wig and a few dabs of spirit gum, he creates Roberto Terranova, an actor who is everything Fitzroy Wynn isn't -- quiet, lyrical, diffidently sexy. Aided by the agent for both Wynns (Adolph Green), Terranova lands the part and Lily, too.

Green's unctuous, much-suffering agent is an old-fashioned figure; like the travelogue-and-cocktail-lounge score (by Szabolcs Fenyes), this character type comes straight out of the movies of the '50s. Screen writer Frank Cucci keeps the dialogue lively and literate, but he doesn't provide any surprises. The tension of "Lily in Love" comes from whether Lily knows Roberto is Fitz, whether Fitz knows she knows, and whether she knows he knows she knows, but Cucci and director Karoly Makk don't make much of it. Here and elsewhere the film lacks an anchoring point of view. What, for example, are we to make of the Wynns' movie-within-a-movie? It seems godawful, but there's no indication that Cucci and Makk intended it that way.

There's a vacuum at the center, and Plummer steps in to occupy it with fortress and garrison. His Roberto is more than makeup -- sweetness suffuses his expression, and there's a telling tentativeness in the way he touches a banister, his little wave of goodbye, his short-strided gait. These are actor's touches -- Roberto is more detailed than real life, and Makk might have done more with this (Lily, surely, has seen enough theater to be tipped off).

As Fitzroy, on the other hand, Plummer booms like a broadside barrage, filling his town house with Elizabethan fustian; he shows us both his talents as a performer and his inadequacies as a man. Fitz possesses real talent (he's not just fuss and feathers), but he doesn't understand it -- his eyes narrowing, mean and petty, he's a bulldog: all instinct. And he holds on to his wife the same way; he knows he wants her, but he doesn't appreciate what a jewel he has.

Smith exudes tremendous charm as Lily; she projects all the intelligence her husband lacks, and she's sexy in a daunting, intellectual way. Just as Lily supports Fitz, Smith supports Plummer, using her lightness to buttress his tragic e'lan. This is no great spiritual odyssey: Fitz is a jerk at the beginning and becomes more of a jerk during shooting (he rails against his wife as a "two-timing strrrrrrrumpet," even though he, as Roberto, is the one who seduced her). And despite the coffee-commercial finale ("Should we have a drink?"), he's still a jerk at the end, with the small but crucial difference that he knows what a bounder he's been.

Plummer's performance is remarkably courageous -- he's boiled down Fitz's humanity right up to the point where we want to see him thrown down a flight of stairs. "It's sad when one's life is winding down," Lily tells Terranova. "Lily in Love" finds the poignance of two people who perform for each other, whose show has come to the end of its run, and whose show must go on.