Another adventure for the third incarnation of the famous timelord. The action this time takes place on Earth, with the Doctor (Jon Pertwee) coming up against all kinds of opposition within the British space program when he attempts to investigate some mysterious messages travelling to and from the returning spaceship Mars Probe 7. When the ship's astronauts arrive back home, suspicious circumstances prevent the Doctor from speaking to them, which only makes him more determined to get to the bottom of the matter.