Nyssa laughed when Tegan told her. ‘It’s not complicated. Here’s an example. What’s the definition of an ancestor?’

Tegan thought for a minute. ‘Well, that’s simple. Your ancestor is anybody who is your father, your mother, your father’s father, your father’s mother, or your mother’s mother or...’ And as she spoke she seemed to see in her mind’s eye a long procession of Nyssa’s ancestors, a line now completely wiped out as a result of the Master’s last evil campaign.

Tegan slammed her fist down on the console and uttered her favourite Antipodean oath. ‘Oh, rabbits!’ She knew in her heart that Nyssa was probably perfectly right, scientifically speaking, but if those were the facts she felt she had a right to protest against them. If the Laws of Nature were unfair they should be subject to appeal in some higher court.

‘Then the TARDIS will just fly on and on until it crashes into something.’ Nyssa made the statement as a scientific fact, and if the idea aroused any emotion in her she didn’t show it. When your whole planet has been wiped out, as Nyssa’s Traken had been, personal danger must seem like light

relief.

Apparently the Doctor did want to communicate, because their departure was further delayed by the lid sliding open a little way, to reveal the Doctor’s face, which looked paler than ever in the sunshine.

He opened his eyes and attempted a smile.

Nyssa bent over him. ‘What is it, Doctor...?’

He blinked in the light. ‘I just wanted to say...’

‘Yes?’ Tegan drew closer too.

‘Er... Forgotten. Never mind, plenty of time.:. It’ll come to me.’

The sunlight seemed to be hurting his eyes, so Nyssa began to draw the lid shut. Then he blinked rapidly and said in a tremulous voice: ‘No, no... Remembered. Thank you. Wanted to say thank you.’

‘And how do we find the Index File?’ A silly thought came into Tegan’s head. ‘Of course, if we had the Index File we could look it up in the Index File under Index File.’ The tension was getting to her; she was thinking and talking nonsense.

But Nyssa took it in her stride. Without pausing at her work at the keyboard she said: ‘Well done. You’ve just discovered recursion.’ Tegan was surprised to be taken so seriously, but Nyssa went on to explain that recursion was a powerful method used to solve some kinds of mathematical problems. ‘It’s when procedures fold back on themselves.’

‘Oh, I don’t understand anything about maths,’ Tegan said. She remembered school exams, and how the wretched figures never seemed to stay still on the paper in front of her.