The Time Lords discover that the Master has stolen secret information about a device called the Doomsday Weapon and realize they need the exiled Doctor's help. They send the TARDIS, along with the Doctor and his assistant Jo Grant, to the bleak planet Uxarieus in the middle of the 25th century where all is not well. What is attacking the colonists and can the Doctor stop his old enemy from finding the dangerous weapon before it is too late?