Cinderella Liberty, playing this weekend at the Harvard Square Theatre, is a weakkneed romantical sketch about a sailor who falls in love in Seattle. Both the movie and its star, professional heartthrob James Caan, have all the depth and charm of a puddle. The only reason for going to see such a harmless piece of pudding is that its co-feature, The Heartbreak Kid, is passably good entertainment. This Elaine May-directed ditty takes a funny but not-too-tender look at a poor schmuck who falls in love, after a fashion, with a shallow American beauty, played to perfection by the evershallow Cybill Shepard. My dentist in Long Island says this movie is unkind to Jewish women, but be that as it may, it's definitely worth seeing. The main feature begins at 2, 5:45 and 9:45.

Mississippi Mermaid, showing at the Brattle until Tuesday, tops the best double-bill in town, Mermaid, like Cinderella Liberty, is a romantic work, but in this case it's romance with a lot of heart and brain behind it. Francols Truffaut, working with Catherine Deneuve and Jean-Paul Belmondo, has created a beautiful motion picture that is in many ways more rewarding than the Antoine Doinel series. Mermaid is playing with another Truffaut flick, The Bride Wore Black starring Jeanne Moreau. This one is relatively lightweight, but it's still an eminently enjoyable bit of story-telling. Shows begin at 5:30 and 9:25.

Play It Again Sam and Carl Reiner's Where's Poppa? are being co-featured at the Orson Welles Cinema I until Saturday. Woody Allen's take-off on the Casablanca theme is not only hysterically funny, it's heart-rending as well. Both Allen and Diane Keaton give lovely performances here. This one is worth watching again and again. Where's Poppa? is usually pretty tasteless and it belabors an already overworn joke to the point of tedium. In all fairness, though, the movie has its moments. Complete shows start at 2:40, 5:50 and 8:55.

Monkey Business, an old Howard Hawks comedy about a scientist who invents a youth potion, moves into the Welles on Sunday. This delightful bit of oldfashioned comedy stars Cary Grant, Ginger Rogers and a youngish Marilyn Monroe. It's being shown with Lifeboat, an Alfred Hitchcock suspense story that's not really up to Hitchcock's standard of excellence. Both movies are inordinately superior to the stuff that comes out of Hollywood nowadays. The double-bill goes on at 2:30, 6:00 and 9:30.

The Harder They Come continues its "smash" run at the Welles, but despite its apparent popularity it's a crummy picture whose only redeeming features are the Jamaican scenery and the reggae. Don't get sucked into this one. You'll be much better off at Cinema III where the Welles is showing Lucia which, despite some boring moments, maintains overtones of brilliance throughout. This one is from Cuba and something of an education. You should probably see it sometime or another. It's playing this week at 4, 7:05 and 10.

Late Night Movies are hard to find in Cambridge, save at the Orson Welles. This weekend's midnight presentations are only so-so. George Harrison's Concert for Bangladesh is okay so far as concert documentaries go; Bruce Lee's Fists of Fury is an action-packed waste of time, suitable only for the hard-core.

King of Hearts is playing for the billionth week at Central Square, but maybe if we ignore it it'll go away.

Z, a milestone of political cinematography, will be shown at the Science Center tonight and tomorrow at 7 and 9:30. If you haven't seen it yet, see it now. If you've seen it only once, see it again. Tonight's film is the first in a series of political movies being shown to raise money for the victims of the Greek dictatorship.