When the curtain slipped down with John Loder and Sylvia Sidney in the third-act clinch of "O Mistress Mine," my throat was a little hoarse from laughing, but I had a vague notion that I had been gypped. For the first two acts of the play I thought I was enjoying not only a genuinely laughable piece, but a comedy which was even sounder for recognizing a human problem and treating it with sympathy. But the final resolution is just a magical blend of cajolery and near-fraud that makes Terence Rattigan's "O Mistress Mine" merely another very funny comedy.

The story is a modern Hamlet with a happy ending. A 17-year old English youth, played by Dick Van Patten, who had the same role in the Broadway version, returns to London in 1944 after spending the blitz period safely in Canada. While he was gone, his newly-widowed mother had fallen in love with Sir John Fletcher, a rich, handsome, married cabinet minister. He finds them living together in luxiuriant--and informal--domesticity.

Son Michael's adolescent super-morality convinces him, as he tells his mother, that Sir John "was vile, and you weakened." To snarl things further, Michael's leftist Canadian "organization" believes Sir John to be a "menace to world industrial reorganization," and just one small step above a Fascist. Sizing up the dramatic possibilities, Michael becomes a moping, moody Hamlet. He believes Sir John murdered his father and accuses his mother of "living in sin" with Fletcher.

Finally Mom decides to give up Fletcher for Michael, and the third act finds mother and son doing nicely thank you at a rather too cheory, peaches-and-cream colored flat in another part of London. When Sir John returns, with a divorce promised, and still in love, Mom at first refuses to marry him, but over a couple of shots of gin with Michael, Fletcher gives the boy ideas for his own love life. Somehow the boy matures and understands, and though the audience is never sure quite how it happened, True Love conquers.

As the irrationally optimistic mother who believes that "there isn't a situation in the world that can't be passed off with small talk," Miss Sidney in completely flighty and helpless; as the remade woman in the last act, she is a depressed lover beneath her maternal gaiety.

Loder's interpretation is faultless. His comic timing is just about perfect, and his lines come out with the right mixture of straight-man and exaggeration to put them over, though occasionally he speaks too fast.

Van Patten had two years with the Lunts to perfect his Michael. His Henry Aldrich-type voice simmers down to a more mature rasp before it gets annoying, and by the time the show is over he is stealing an occasional scene from his elders. Given his fantastic American-Canadian-English lines that come up with such expressions as "jolly swell," Van Patten doesn't waste a syllable and is master of the double take. He has the difficult duty of acting a ludicrous person with a concrete and serious problem and he performs it without slipping into schizophrenia.

In the smaller roles, Edna Preston bustles as the perpetual Cockney maid is expected to, and Mary Mace is proper and prim as Sir John's secretary. Maxine Sheppard looks natural in the cynical, sophisticated--and bitchy--role of Lady Fletcher.

I didn't see the Lunts do this play, and it's hard to say how much of the staging is theirs and how much director Harald Bromley added, but the effect is well-knit and unobtrusive. I suspect the Lunts' edge over the Sidney-Loder duo was in making every shot count; some humorously intended lines in the present rendition just can't lug their point across the footlights. But that still leaves enough laughs and satire and embarrassing encounters of the "Uh-oh, look who's here" type to amuse you for a couple of hours--so long as you don't expect to remember the play more than a week.