The Greek army has besieged the city of Troy for ten years. The TARDIS materializes outside the city, distracting Hector (Alan Heywood) and allowing Achilles (Cavan Kendall) to kill him. When The Doctor emerges, Achilles believes he is Zeus and brings him to the Greek camp. Agamemnon (Francis de Wolff) wants The Doctor to help fight the Trojans, though suspicious Odysseus(Ivor Salter) thinks he’s a Trojan spy. Vicki and Steven remain in the TARDIS. Vicki is injured, but Steven goes to find The Doctor. Odysseus captures him, but The Doctor, as Zeus, convinces the Greeks to spare him until the next day.
The TARDIS has been taken into Troy as a gift to King Priam (Max Adrian) , though Cassandra (Francis White) warns against it—she has dreamt that the Greeks will leave a dangerous gift and demands that it be burnt. But Vicki’s emergence from the TARDIS is taken as a sign; she is called Cressida and becomes a favourite at court. Cassandra considers her a rival prophet but her handmaiden Katarina (Adrienne Hill) befriends her.
Paris (Barrie Ingham) calls for battle with Achilles, but Steven calls himself Diomede and takes Achilles’ place, hoping to be captured and taken into the city, where he hopes to find Vicki. The Doctor gives Odysseus the idea of the Trojan Horse and joins the soldiers inside. When he and the Greeks emerge, he finds Vicki and Steven with Katarina. Vicki has fallen in love with Troilus (James Lynn) and remains behind. Steven is wounded and Katarina, caring for him, takes him into the TARDIS. Odysseus threatens The Doctor, so he dematerializes the TARDIS, taking Katarina with him along with the injured Steven. She believes she has died, and The Doctor is taking her to the afterlife.
Margaret O’Brien’s departure as Vicki was a shock to her; she returned from vacation to discover that she had been written out. They pretty much did the same to her replacement, Adrienne Hill, when they discovered that a handmaiden from Troy was not a good TARDIS companion—everything had to be explained to her. Then they tried the same trick on William Hartnell.