UNIT invests Mars Probe 7 and the Doctor joins them.  Space Control has lost contact with the astronauts seven months ago when it left Mars.  Now the rescue mission has run into similar problems.  The second ship return and the astronauts are kidnapped just after landing.  Liz Shaw notices that the Geiger counter is pegged.  The crew of the retuning ship's occupants were alien ambassadors and not the original astronauts.

The Doctor takes a rocket and docks with Mars Probe 7 which is still in space.  An alien spaceship takes him aboard where the captin threatens to destroy the Earth unless their three ambassadors are released.  He finds the original astronauts on board and unharmed.  The Doctor is released and returns to Space Control.  He finds that the ambassadors have been kidnapped as part of a plan by General Carrington to discredit the aliens and cause war with them.  The Doctor and UNIT are able to arrange for the exchange of ambassadors for astronauts and avert the war.