A previously unknown planet is discovered within our solar system, orbiting on the far side of the sun exactly opposite the position of the Earth, and at precisely the same speed. The European space agency Eurosec, headed by Jason Webb (Patrick Wymark), whose solar probe made the discovery, decides to send a manned mission to investigate, teaming America's top astronaut Glenn Ross (Roy Thinnes) and British astro-physicist John Kane (Ian Hendry). Their voyage aboard the space vehicle Phoenix is supposed to take six weeks, but when the ship returns to orbit in only three weeks -- ending in a crash of their landing vehicle that kills Kane -- Eurosec can only conclude that Ross has engaged in some sort of sabotage. The astronaut is at a loss as to how they could have done a round-trip in just three weeks, until he makes a startling discovery -- that everything that he sees, from the layout of rooms and buildings to all of the writing around him, is reversed, left to right and right to left. It takes Ross, amid his confusion, to arrive at the only possible conclusion -- that he and Kane did, indeed, journey to the new planet, and that world is a duplicate of Earth (and visa versa) down to the last molecule, a perfect mirror-image; and that world dispatched its own mission, with its own Ross and Kane. He and Webb, and Eurosec, scarcely have time to absorb the implications of this discovery -- if true -- as they prepare for a return flight for Ross, despite enormous risks and some potentially very dangerous unknowns in getting him back to the Phoenix.