The TARDIS materializes on 21st Century Earth.  They find that society is totally reliant on T-Mat.  This is a matter transmision device that teleports matter to anywhere in the world.  The system is malfunctioning and the TARDIS crew agree to pilot an old rocket to the Moon relay station to determine the problem.

When they get there they find Ice Warriors.  They are preparing for an invasion of Earth and are using the T-Mat to send seed pods to different ares of Earth.  These pods are transforming the planet with fungus to be more like Mars.  The Doctor and his compaions use the T-Mat to get back to Earth.  They discover that the pods are susceptible to water and they have the weather control bureau cause a downpour.  They return to the Moon and send the invasion fleet into orbit around the Sun.