The Kingfisher, by William Douglas Home. Directed by Lindsay Anderson; setting by Alan Tagg; costumes by Jane Greenwood; lighting by Thomas Skelton. Produced by Elliot Martin, Hinks Shimberg and John Gale.

With Rex Harrison, Claudette Colbert and George Rose. At the National Theatre through March 15.

When Rex Harrison, who has just downed his third or fourth brandy, lures Claudette Colbert, who has put away a like number of cremes de menthe, to a blanket under an old beech tree, it is, naturally, with marriage on his mind.

We know it. She knows it. So she skips across the lawn, drops gracefully into perfect proposal-receiving posture and invites him to speak his piece.

"Will you marry me?" asks Harrison pleadingly, with the same big string instrument of a voice that has been hiding all these years in that deceptively normal-looking larynx (which should be insured by Lloyds for several million pounds).

Colbert, whose good looks and nimble delivery seem to have been presented by the same benificent muses, has been turning him down for an act-and-a-half. Now, at last she submits -- and very sweetly, too -- but she can't resist a little swipe at the tree, the blanket and all the other atmospheric rigmarole. "What a sentimentalist you are," she says.

"Well," says Harrison, "it did the trick, didn't it" and looking as pleased with himself as a cat lapping up a pool of cream, he makes ready for some post-acceptance snoozling. But his fiancee, apparently regarding her part of the night's business as over, suddenly rises and with a brisk "And now I'm off to bed," leaves him thoroughly nonplussed, locked in an extremely funny embrace with thin air.

Is that revealing too much of the plot of "The Kingfisher," which opened at the National Theatre last night? No, you could know everything there is to know about William Douglas Home's play and still enjoy being escorted through it by this nimble pair. You could know absolutely nothing and still find it every bit as familiar as an overstuffed chair.

First produced several seasons ago in London with Ralph Richardson and Celia Johnson, "The Kingfisher" is a three-character romantic comedy about an old bachelor making one last stab at winning the hand of the woman he has loved for 50 footloose years.

Home, an Englishman (whose brother Alec was prime minister in 1963 and '64), has been practicing his craft nearly as long as Harrison and Colbert have been practicing theirs, but it is no disservice to this particular effort to slide past the details and simply say the "The Kingfisher" is what is called a "vehicle." And it does what any vehicle should do: It delivers the goods.

As written, the part of Evelyn, the just-widowed Lady Townsend, places no great physical demands on the actress who plays her.But Colbert makes several galloping exits that leave us in no doubt that she could wrestle an alligator if asked. At the self-confessed age of 76, she is not only a chronological wonder but, as ever, one very crafty comedienne.

Colbert remains elegance personified -- but elegance spiked with an agreeable dash of the uncouth. Recalling Evelyn's marriage to a Wodehousian dimwit named Reggie, she confesses that he never made her happy, not in all their 50 years together. "No, I never loved him," she tells Harrison as her once-and-future beau, Sir Cecil.

"But you had two children," says he, looking naughtily aghast. "How'd you manage that?"

Not to be outdone, Colbert gives him a look that drips with sarcasm and replies, "I shut my eyes and thought of you."

Harrison, perhaps shaping up for next year's planned revival of "My Fair Lady," indulges in an occasional dance step himself, but generally prefers to work with the smaller muscles of his eyebrows and fingers. The part of Sir Cecil, "the best-selling, the most famous author in the British Isles," is the sort of thing Harrison could walk through in his sleep -- and indeed there are times when he seems to be asleep. But even the way he covers a flubbed line, cranking his vocal chords in a sustained mumble until the right phrase comes back to him, is supremely graceful.

To leave co-star George Rose for last may seem grossly unfair, but he, after all, has been acting only 35 years or so, which makes him a mere tyke in the present company. Looking as fussy as a maiden aunt, his mouth pursed every downward, his hands clasped together and his feet primly pointed 90 degrees apart, Rose is a perfect cutup as Hawkins, Sir Cecil's faithful valet.

Hawkins is always hinting the worst about Evelyn and her designs, and at one point, when Sir Cecil insists that "she's not that kind of girl," the valet coldly responds: "God only made one kind of girl!"

As you may have gathered by now, "The Kingfisher" has its acerbic moments.

Evelyn's departed husband, as she reveals in a rather disrespectful tone, died playing golf.

"Oh, what hold did it happen on?" asks Sir Cecil.

"Seventeen," she says.

"He almost made it, didn't he?" is Sir Cecil's retort.

Home's play also contains a host of double-entendres and mildly shocking gags about sex as well as death, but it is basically light as a dish of meringue and as harmless as a circus bear with a touch of arthritis.

Here, in short is proof that they do make 'em like they used to, so don't let anyone tell you they don't.