When a mysterious object lands near the Nuton power complex, UNIT investigate and encounter the Axons, a group of aliens who offer the mineral axonite, which can duplicate matter, as a gift.

The Doctor and Winser begin to analyse axonite while Chinn plots to secure its use exclusively for Britain.

The Doctor and Jo are being held prisoner by Axos while Chinn is forced to organise the worldwide distribution of axonite and the Master returns to the power complex to try and steal the TARDIS.

The Doctor and Jo escape from Axos but then the Doctor offers the Master a deal; if he helps him repair the TARDIS, they can flee Earth together.

An unnamed world, an unknown time. A vast mining vessel trawls a sea of mineral sand for valuable ores, the tiny human crew served by an army of humanoid robots. But as the Doctor and Leela arrive, one of the crew is found murdered... and if they didn't do it, who did?

The Doctor and Leela are suspected of murder. But the real foe is using robots to cut down the crew – and even sabotage the sand miner itself!

One of the crew is the insane Taren Capel, who is using robots to carry out the murders. The Doctor discovers that there are also others who aren't what they appear to be either.

The Doctor and the surviving humans fight for their lives against Taren Capel and his growing army of deadly robots.