When a pioneering space probe returns to present-day Earth, the Doctor and UNIT discover that it is empty. What has happened to the three astronauts who went up into space, and where are they now? This is the original TV soundtrack, with new linking narration by a member of the cast, of the seven-part adventure from 1971. This BBC production stars Jon Pertwee as the Doctor, with Caroline John as his companion Liz Shaw. Nicholas Courtney stars as the Brigadier.