The TARDIS arrives at the South Pole in December of 1986.  General Cutler, in command of the Space Tracking Center there, is trying to talk down a manned space capsule that is in trouble.  The Doctor determines that the problem is from the another planet's gravitational pull.  It has entered the solar system and is heading straight for Earth.  After the spacecraft lands, the base is invaded Cybermen.  Mondas is the name of the planet and it starts to drain energy from Earth.  It was once Earth's 'twin planet' but was somehow sent off into space and has just now returned to the solat system.  The Cybermen are to take the humans to Mondas for conversion but the people from the base refuse.  The planet Mondas disintegrates when it absorbs too much energy from Earth.  All the remaining Cybermen collapse and die since they drew their energy from their planet.  The Doctor, weakened during this adventure, returned to the TARDIS with Ben and Polly.  Here he regenerates into a new body, much to the surprise of his companions.