With the TARDIS disabled for repairs, the Doctor is powerless to act when Romana is kidnapped by a battleship en route to Skonnos and the formidable Nimon.

On Skonnos, Romana and the Athenian youngsters are forced into the labyrinth of the power complex to meet their fate at the hands of the Nimon.

Trapped inside the power complex, the Doctor and Romana begin to suspect that the Nimon is only the first of a full scale invasion.

On the dying world of Crinoth, Romana discovers the truth behind the Nimon's empty promises. The Doctor must find a way to shut down the power complex on Skonnos.