A is for... Astra

Princess Astra was the sixth child, of the sixth dynasty, of the sixth royal house of the planet Atrios. She was the last of her line. Atrios had been at war with its twin world of Zeos for many years. Astra had very little authority - acting more as a popular figurehead whilst the planet was run by her Marshal. Sickened by the constant death and destruction, she and her lover - a doctor named Merak - had planned to set up secret peace negotiations with the Zeons, but they found that their messages went unanswered. Suspecting her treachery, the Marshal decided to get rid of her by shutting her up in a heavily irradiated section of the underground city. This actually housed a transmat station, and Astra was captured by the Shadow, servant of the Black Guardian. The Doctor and Romana were seeking the sixth and final segment of the Key to Time on Atrios, and found that the tracer tended to follow Astra's movements. They suspected that she might possess the disguised segment. Astra was taken over by the Shadow and used in his plans to capture the Doctor and gain the first five segments.
On checking Astra's medical records, Merak discovered a genetic anomaly. It transpired that Astra herself was the Sixth Segment. She herself sensed this, and was transformed into the segment on touching the tracer.

It was his disregard for her death that led the Doctor to suspect that the White Guardian, when he turned up to claim the Key, was really the Black Guardian. The Doctor re-scattered the Key segments, and so Astra was restored and reunited with Merak.
Her likeness was later adopted by Romana when she regenerated.

Played by Lalla Ward. Appearances: The Armageddon Factor (1979).

• Mary Tamm had resisted all efforts by the production team to agree to do a second year on Doctor Who. Tamm was one of those who recommended Ward to take over from her, having seen how well she got on with Tom Baker.
• Astra's parents must have only just died - else why is she not a queen? A coronation would surely have been a great morale boost for the Marshal.
• Astra's restoration suggests that all of the Key segments returned to the forms initially found by the Doctor and Romana - making them susceptible to the Black Guardian surely? The Doctor no doubt thought of that when he scattered them - treating Astra as a special case whilst the other five pieces got new forms. With total power over the entire universe, he may even have willed it that she was no longer a key segment.