"Woman of the Year," first celluloid of Katherine Hepburn's since she swept stage and screen audiences off their feet with "Philadelphia Story," is something, though not too much, of a disappointment.

It thrives on tricks, surprises, and the paradoxically brittle warmth of Miss Hepburn's acting. For the firs half of the picture the tricks are new, the surprises come fast and furiously, the acting adds tempo and supplies authenticity. Then the script writers seem to run out of ideas, and begin to fall back on old, familiar slapstick. You keep remembering how well; everything had started out and looking for a twist, a turn, a climax that doesn't come. When the lights turn up you go out laughing, but a little sorry that the most promising comedy in a long time had to flatten into just a good comedy.

Career woman marries family man, sees the light, and patches up: that's a skeleton of a plot we know so well. But it's doctored up so that the career woman is Tess Hardy, a diplomat's daughter turned political columnist (sort of a combo of Dorothy Thompson and Mrs. Roosevelt all rolled into the frame of La Hepburn), and the family man is sports writer Sam Craig (better known as Spencer Tracy on M.G.M.'s payroll). Up until their wedding night, the picture hits all the heights of humor and contrast you could ask for. His friends are the worn-out prizefighters, gamblers, and reporters that hang around Joe's bar; her friends are prominent diplomats, statesmen, and political prime-movers. His line is sports; her's is "the problems of the day." His language is Bill Cunninghamese; her's is every foreign tongue worth gargling. He takes her to see a baseball game from the press box; she asks him to one of her international cocktail parties. On their wedding night her big wigs and his sports cronies clash in a bedroom scene that takes every cake Hollywood ever baked. Up to this point, the picture is fast moving, crisp and new. The last half loses the delights of contrast and lapses into the faked-up tricks of mechanical slapstick that provide only artificial life to an old plot.

"Boy meets girl" is great. "Boys loses girl" and "boy gets girl" are only mediocre. But "Woman of the Year" might still rate the comedy of the year.