Adric was a native of Alzarius who, following the death of his elder brother, stowed away in the Tardis. When Romana and K9 left, the next two stories gave time to demonstrate the similarities between the youth and the Doctor. Like his mentor, Adric was a genius who rebelled from his own society, thirsting for experiences, and was inquisitive in the extreme. But while he wore a badge for mathematical excellence, he was still immature.

Nyssa came from a planet that embraced tranquillity with science, and as such she was a serene, warm but scientifically-minded young woman who always kept a level head no matter what was happening around her.

However, Adric considered Nyssa to be "a girl", simply because she came from a protected upbringing. Worse still, Adric was very sexist which immediately ran him into trouble with the third companion, Tegan Jovanka, who completed the team around the new fifth Doctor. "That's the trouble with women," Adric once complained. "Mindless, impatient and bossy."

When Peter Davison took on the role of the Doctor, the ready-made crew fitted into place around him. Nyssa was the soothing intelligence that the Doctor could relate to. Adric also had a fantastic mind, but his juvenile streak irritated the Time Lord, causing him to be impatient with the boy. Tegans blind Ozzy instinct made itself known to them all like a bulldozer makes itself known to a block of flats.

The intellectual friction between the Doctor and Adric reached a head in Earthshock. The boy wanted his mentor to prove he cared for him. Adric asked what he knew was practically impossible - for the Doctor to take the Tardis back to E-space and return him home. After a big argument the Doctor eventually agreed. Adric hadn't meant it, the Doctor's agreement had been proof enough. He had spent many long sessions talking and learning more about mathematics in his times with the Doctor, and in the end it was his own desire for perfection and excellence in his work that killed Adric.

He attempted to defuse a Cyberbomb, and was working on the final solution to the mathematical codes when the lock was destroyed and he perished in a terrible explosion far in the prehistoric past of the Earth.

Nyssa had lost her father, her world, seen the Doctor lose one of his lives, and now had witnessed the death of one of her close friends. After such a catalogue of death any human would have cracked up. But with Nyssa, it only strengthened her sense of good, and prepared her for a much harsher environment. Terminus was the Lazer colony that became Nyssa's home, where she left the Doctor and Tegan to try and help save the dying by stabilising the cure.

He tries to, but he knows that this is no one-in-a-million chance meeting. Something made Tegan come to this place and time. Something in her subconscious. The conclusion being that she is still not free of the Mara yet, and this leads to Snakedance.

Season 20 was to prove to be a crossroads in the programme's history; when the title character would stop being a pioneer explorer and start being a magnet to all past enemies. But season 20 also introduced an aspect of the Doctor's character which would be picked up again in the McCoy years.

In the way that the Doctor would have known about the Mara, so he also knew the nature of his next companion - the turncoat refugee called Turlough. Yet he diced with him. Like Ace in later years, the Doctor turned the Black Guardian's pawn against him by offering the boy understanding; and by doing so winning what the Black Guardian had called "the game" (a direct quote from the beginning of Enlightenment).

Tegan and Turlough's love/hate relationship was highly original. He thought that he always had to prove himself to Tegan and he resented having to do so. She thought he was a weakling - he should be a "macho Australian guy" sort of thing.

Experience of death and destruction had strengthened Nyssa's resolve to save lives. Her background meant she had the inner strength to handle it all.  Not so Tegan. Resurrection of the Daleks. The resurrection of murderers and the death of innocents.

The Doctor thought it through logically. If the innocents were to survive then the murderers must die, and the survival of the Daleks relied on Davros. So Davros must die.. It was logical. The first Doctor would have had no qualms about doing it. But from the fifth, more "human" Doctor, this was quite a shock. And when Tegan looked at the carnage and pointless death around her, it was too much of a shock. In the most emotionally-charged scene the programme has ever done, Tegan said goodbye to the Doctor, adamant that she couldn't carry on travelling with him.

She ran off, but returned to see the Tardis leave without her. Alone, in London, this air stewardess realised that the most incredible flight of her life was now over.

Brave heart, Tegan ...