The TARDIS is almost caught in a web-like material in space but escapes to materialize in the London Underground.  The tunnels are being taken over by the same web-like stuff and by the Yeti.  It has been forty years on Earth since the TARDIS crew were in Tibet.  Professor Travers has accidentally reactivated on of the Yeti which allows the Great Intelligence to make another attempt at conquering Earth.

The TARDIS crew work with a Colonel Lethbridge-Stewart to battle against the Great Intelligence.  The Great Intelligence attempts to drain the Doctor's mind but he sabotages the device.  He tries to drain the Great Intelligence's 'mind' but he is cannot complete the process.  The Great Intelligence is defeated.