A weird green object has appeared in outer space! The United States Government requests Tom Swift Jr. to investigate this menace to earth's radio communications and threat to national security. But the Orb exerts an electromagnetic power that endangers Tom's spaceship and he is forced to return to earth.

Since it is impossible to land men, Tom designs robot astronauts, equipped with his new three-dimensional TV cameras as "eyes," to probe the Orb. While working feverishly to perfect his Video Viking robots, the young inventor discovers that the Orb has succeeded in communicating its hostility to a ruthless scientific group on earth and has ordered the leader of Q to destroy Swift Enterprises.

After vanquishing his earth enemies, Tom launches his robots. The startling three-dimensional images of an unheard-of phenomenon that the Video Vikings send back during their probe of the Orb result in Tom's making another attempt at a manned landing.

The dramatic showdown with the master brain of the Green Orb climaxes one of the most daring space exploits in the young scientist-inventor's thrilling career.