England, the 12th century. Robber baron Irongron is recruited by Linx, a Sontaran warrior whose starship has been forced down on Earth. Linx is forced to use a time-travel device to steal resources and scientists from the future to repair his ship, but in doing so involves UNIT and the Doctor...

The Doctor tracks the missing scientists to Irongron's castle, where Linx is using them to repair his spaceship and construct advanced weapons for Irongron.

The Doctor escapes the castle with Sarah and Hal and must persuade Sir Edward to trust him before Irongron launches an attack on them.

The Doctor attempts to return the scientists to their own time, before Linx tries to use his spaceship and destroys the entire castle.