If astonishment is a necessity in the theater, mimes must constantly be at wits' end. Theirs is the most traditional of the performing arts. Plexus, the pantomime trio performing at the Source on 14th Street NW through Saturday, has devised at least two solutions to this dilemma of creating surprise within strict rules.

After a couple of routine pieces, the first solution was stated in three brief but brilliant encounters with "Beasts." One of the mimes, wearing a cow's head, advanced on the audience sharpening a carving knife. Blackout.

Another mime, this time with a horse's head, limped halfway across the stage on a crutch, stopped, pulled a pistol out of a pocket and held it to his head. Blackout.

For the finale, a chase. A detective, while pursuing a bloodhound who has broken out of jail, held a pair of binoculars glued to his animal face. Finally he lowered them and we saw that this was a beast without eyes.

These flashed fables made one sit up with a sense of shock. They were over before wearing out their terse ideas. "Quest," with a protagonist from a Balthus painting who is dissatisfied with his face to the point of no return, was another concise, pointed charade.

Character development supplied novelty in the absurdities of a circus act. Jyl Hewston, Robert Morse and Joe Mori were a Russian family outfit that juggled and balanced ad infinitum and knew too few other tricks. But the longer and triter the piece became, the more the trio seemed like real people who had stepped from one of Gogol's books.