When a saucer sighting starts parents on a ""logical explanation"" you know something's astir. . . in the gismo seventh grader Jerry Cole finds in an alley and hooks up to a crystal set and a telephone. And by the stars, it's the lost communications system from spaceship XR, transmitting direct to Throalian Monaal who'd have it back (the earth as usual being unready). One after another rendezvous goes awry--Jerry's parents' nix on tired eyes is no help although his pesty sisters are--until, with his and pal Ron's families in the know, Monaal thwarts the elders' plan to turn the gismo over to the government and prods Jerry to make one of his own. Science fiction shrunk to domestic comedy (little sister Dodie prattles with the spaceman about his daughter after Jerry can't get anything out of him), which the Leonard Shortall illustrations should pep up. Functional if no marvel.