Transmitted originally: 13 March 1971 to 3 April 1971.
The Doctor is suspicious, and his doubts are proved right: Axos, Axonite and the Axons - whose true appearance is hideous - are all part of a single parasitic entity brought to Earth by the Master to feed on the energy of the planet. The Doctor manges to materialise his TARDIS, with the Master on board, at the centre of Axos. He offers to link the two craft together to create a giant machine, on the condition that Axos helps him to take revenge on the Time Lords for his exile to Earth. This is merely a trick, and Axos is locked in a time loop from which it can never escape. The Doctor returns to Earth in the TARDIS, while the Master also manages to escape.
The interior of the TARDIS is seen for the first time since 'The War Games'.
The line descibing how Axos's arrival has caused freak weather effects was inserted after the unpredictable weather experienced during filming.