The evil MASTER has stolen the Time Lords' file on the horrifying DOOMSDAY WEAPON with which, when he finds it, he can blast whole planets out of existence and make himself ruler of the Galaxy!

The Time Lords direct DOCTOR WHO and Jo Grant in TARDIS to a bleak planet in the year 2471 where they find colonists from Earth under threat from mysterious, savage, monster lizards with frightful claws! And hidden upon this planet is the DOOMSDAY WEAPON for which the MASTER is intently searching ...