Tegan steers the TARDIS to the planet Manussa, without meaning to - but why? And is it a coincidence that the planet is about to celebrate the 500th anniversary of the expulsion of the insidious Mara?

The Doctor talks to Ambril and Chela to try and stop the 500th anniversary celebrations. A possessed Tegan finds herself in Dugdale's Hall of Mirrors to where she summons Lon. The Doctor and Nyssa return to the caves.

The possessed Tegan and Lon try to find the whereabouts of the Great Crystal. The Doctor finds himself in prison, with only Nyssa to try and get him out. Chela tells the Doctor tales of Dojjen.

The Doctor and the original snakedancer Dojjen work to rid the planet of the Mara.