Possessed Tegan and Lon try to find the location of the Great Crystal. The Doctor finds himself in prison with only Nyssa to get him out while Chela tells the Doctor about Dojjen and gives him Dojjen's journal.

The Black Guardian strikes a deal with a school student named Turlough to kill the Doctor. He promises him that he will take him back to his home planet if he does the job. Meanwhile The Doctor's Tardis is trapped by a hidden device near Turlough's school.