SLOW AND STEADY wins the race in "Turtle Diary," a quietly optimistic chamber comedy by Harold Pinter, whose script is delicately acted by Ben Kingsley and Glenda Jackson as two lonely Londoners living lives of quiet desperation and all that.

They share an interest in the 30-year-old giant green sea turtles at the London Zoo, and, oh-so-cautiously, determine to return the grand creatures to the sea. Their gentle act of civil disobedience (turtle terrorism?) enables the two of them -- it has to be said -- to come out of their shells just a little.

Kingsley plays William Snow, a bookshop worker with a submerged rebellious streak -- his main frustration is with a loutish neighbor in his rooming house who leaves bathtub rings and refuses to clean the rangetop. Jackson chooses a tight-lipped assortment of tics and winces for Neara Duncan, arim, wary best- selling author of children's books.

Their caper goes swimmingly, of course, but success, in fact, seems beside the point. "Turtle Diary" is a lament for the sorry state of 20th Century Urban Man. Pinter's brittle London dialogue is enveloped by intrusive street noise, the camera creeps through claustrophobia-inducing hallways, and the skies remain pewter-gray and grim. Then the turtlenappers hit the road and the sun gleams, the sea glistens, the turtles take off (they look grateful) and life begins to look as though it has possibilities again.

Pinter makes a funny cameo appearance as a bookshop customer interested in sequels, and his uncommonly sweet script is full of Pintery splinters of dialogue, with everyday wit punctuated by awkward silences and pauses.

There are a lot of shots of Snow and Duncan pondering and fretting, and Pinter and director John Irvin sometimes get a little hamfisted with the symbolism. But more often their simple conceits work delightfully well. Irvin fills the film with simple visual jokes.

And the change in Snow and Duncan -- a relaxing, lightening and visible sense of achievement -- is shared by the audience. An exceptional adventure movie, "Turtle Diary" certainly takes its own sweet time, but the patient will be rewarded.

TURTLE DIARY (PG) -- At area theaters.