In this last installment of the mysterious and exciting Black Guardian/Turlough trilogy, immortal Eternals -- who are racing to achieve enlightenment -- challenge mortal Ephemerals, and we learn being an Ephemeral is better. But once again, the nefarious Black Guardian appears, and the Doctor is suddenly in a race to save his own life. Peter Davison, Mark Strickson, Janet Fielding and Cyril Luckham star.