In their search for the 3rd segment of the key to time, the Doctor and Romana land on Earth at a Stonehenge type site where worshipers are engaged in their practice and people are being killed by unknown causes. While en route the Doctor and Romana hear a strange voice that says 'Beware the Black Guardian".

The Doctor transfers to Hyperspace to save Romana from her imprisonment and has a run in with the Megara Justice machines during his search.