He looked up as she came in. 'Oh, hello, Barbara. Not gone yet?'

'Obviously not.'

Ian groaned. 'Oh well, ask a silly question!' Barbara was frequently sharp-tongued, especially when tired or worried.

'I'm sorry,' said Barbara quickly.

'It's all right, I'll forgive you - this time.'

She perched wearily on a laboratory stool. 'It's just that something's worrying me rather. I don't know what to make of it.'

It was unlike her to confess helplessness, and Ian was immediately concerned. 'What is it? Can I help?'

'Oh, it's one of the girls. Susan Foreman.'

Ian's eyes widened. 'Susan Foreman! You find her a problem too, do you?'

'I most certainly do!'

'And you don't know what to make of her?'

Barbara shook her head.

'Me neither,' said Ian ungrammatically. He looked thoughtful for a moment. 'How old is she, Barbara?'

'About fifteen.'

'Fifteen!' Ian ran his fingers through his already untidy hair.

'Do you know what she does? In my science classes, I mean?'

'No, what?'

'She lets out her knowledge a little bit at a time!' he said explosively. 'I think she doesn't want to embarrass me. That girl knows more science than I'll ever know. Is she doing the same thing in your history lessons?'

'Something very like it.'

'Your problem's the same as mine then? Whether we stay in business, or hand the class over to her...'

'No, not quite.'

'What then?'

Barbara Wright leaned forward on her stool. 'I'm sorry to unload all this on you, Ian, but I've got to talk to someone about it. I don't want to go to anyone official in case I get the girl into trouble. I suppose you're going to tell me I'm imagining things?'

'No, I'm not.' Ian turned down a Bunsen burner and began washing test tubes and glass Petri dishes in the laboratory sink, stacking them neatly in racks to dry. 'Go on.'

'Well, I told you how good she was at history? I had a talk with her, told her she ought to specialise. She'd be a natural for a university scholarship in a year or two, Oxford or Cambridge if she wanted.'

'How did she take it?'

'She was cautious about it, but she seemed quite interested...'

Barbara paused. 'I told her it would mean a good deal of extra study, offered to work with her at home. The whole idea seemed to throw her into a kind of panic. She said it would be absolutely impossible because her grandfather didn't like strangers.'

'Bit of a lame excuse, isn't it?' said Ian thoughtfully. 'Who is her grandfather anyway? Isn't he supposed to be a doctor of some kind?'

Barbara nodded. 'Anyway, I didn't pursue the point, but the whole thing seemed to upset her somehow. Since then, her homework's been, I don't know, erratic - sometimes brilliant, sometimes terrible.'

'Yes, I know what you mean,' said Ian. 'She's been much the same with me.'

'Anyway, I finally got so worried and irritated with all this that I decided to have a talk to this grandfather of hers, and tell him he ought to take a bit more interest in her.'

Ian smiled to himself. It was very typical of Barbara to get herself worked up and go marching off to lecture some perfect stranger on his family responsibilities.

'Did you, indeed? What's the old boy like?'

'That's just it,' said Barbara worriedly. 'I got her address from the school secretary, 76 Totters Lane, and I went along there one evening.'

By now Ian was busily preparing a microscope slide from some mysterious solution in one of his test tubes, head bent absorbedly over his work.

'Oh Ian, do pay attention!' snapped Barbara.

'I am paying attention,' said Ian calmly. 'You went along there one evening. And?'

'There isn't anything there. It's just an old junk yard.'

'You must have got the wrong place.'

'It was the address the secretary gave me.'

'She must have got it wrong then,' said Ian infuriatingly.

'No, she didn't. I checked next day. Ian, there was a big wall on one side, a few houses and shops on the other, and nothing in between. And that nothing in the middle is the junk yard, 76 Totters Lane.'

Ian finished his slide and put it to one side. 'Bit of a mystery...?

Still, there must be a simple answer somewhere. We'll just have to find out for ourselves, won't we?'

'Thanks for the we,' said Barbara gratefully. She looked at her watch. 'The poor girl's still waiting in my classroom. I'm lending her this book on the French Revolution.'

Ian looked at the bulky volume. 'What's she going to do -

rewrite it? All right, what do we do? I doubt if it'll do any good to start firing questions at her.'

Barbara shook her head decisively. 'No, what I thought we'd do is drive to Totters Lane ahead of her, wait till she arrives, and see where she goes.'

'Got it all worked out, haven't you?' said Ian admiringly. 'All right!'

Barbara looked hesitantly at him. 'That is - if you're not doing anything...'

'No, I'm not doing anything,' said Ian reassuringly. 'Come on, let's go and take a look at this mystery girl.'

They went out of the laboratory, along the corridor, and into the classroom, which was empty except for Susan Foreman and the sound of rock and roll blaring from her transistor radio.

Barbara raised her voice. 'Susan?'

Susan looked up. 'Sorry, Miss Wright, I didn't hear you come in.'

'I'm not surprised.'

Susan's face was alight with interest. 'Aren't they fabulous?'

She looked every inch your average normal teenager, thought Barbara. But she wasn't. She wasn't...

'Aren't who fabulous?'

'John Smith and the Common Men. They've gone from number nineteen to number two in the charts, in just a week.'

'John Smith is the stage name of the Honourable Aubrey Waites,' said Ian solemnly. 'It's not so fashionable to be upper class these days. He started off as Chris Waites and the Carollers, didn't he?' Ian Chesterton wasn't exactly a pop fan, but he found it helped to keep in touch with the interests of his pupils, so he knew what they were talking about, at least some of the time.

Susan looked admiringly at him. 'You are surprising, Mr Chesterton. I wouldn't have expected you to know things like that.'

'I've an enquiring mind,' said Ian. 'And a sensitive ear,' he added drily.

'Sorry,' said Susan, and switched off the radio.

'Thanks!'

Susan looked at the bulky volume under Barbara Wright's arm.

'Is that the book you promised me?'

Barbara handed it over. 'Yes, here you are.'

'Thank you very much,' said Susan politely. 'I'm sure it will be very interesting. I'll return it tomorrow.'

'That's all right, you can keep it until you've finished it.'

'I'll have finished it by tomorrow,' said Susan calmly. 'Thank you, Miss Wright, goodnight. Goodnight, Mr Chesterton.'

Ian looked thoughtfully at her. There was something strange about Susan Foreman, despite all her apparent normality. Her speech was almost too pure, too precise, and she had a way of observing you cautiously all the time, as if you were a member of some interesting but potentially dangerous alien species. There was a distant, almost unearthly quality about her...

'Where do you live, Susan? I'm giving Miss Wright a lift home, and there's room for one more in the car. Since we've kept you late, it seems only fair you should get a lift as well. It'll soon be dark.'

'No thank you, Mr Chesterton. I like walking home in the dark.

It's mysterious.' Susan put the radio and the book in her bag and turned towards the door.

'Be careful, Susan,' said Barbara. 'It looks as though there'll be fog again tonight. See you in the morning.'

'I expect so. Goodnight.'

The two teachers waited till her footsteps died away and then Ian took Barbara's arm. 'Right - car park, quick! We are about to solve the mystery of Susan Foreman!'