That was Tuesday night. On Wednesday morning Mr Arthur and Monsieur Francis were happy men. Europe liked the new Margarita - everything in life was good. The next opera night was Friday. It was Faust again, but this time - with Isabella Fernández Carriedo singing Margarita.

By Wednesday afternoon they were not so happy. A second letter arrived for them - from O.G.

Why don't you listen to me? I am getting angry. Leave Box 5 free for me. And where are my 20,000 euros? On Friday Héderváry must sing Margarita again. She is now the best singer in Europe. Isabella Fernández Carriedo cannot sing - she has a very ugly voice, like a toad.

Remember, I am a bad enemy. OG.

'So, Francis, is this still a joke?' Mr Arthur was steaming. 'What are we going to do now, hm? Is O.G. the director here, or are we?'

'Don't be so cranky, Arthur,' said Monsieur Francis tiredly. 'I don't know the answers. Let's talk to Madame Zwingli, the doorkeeper of Box 5. Perhaps she can help us.'

But Madame Zwingli was not helpful. Madame Zwingli was not afraid of ghosts, and she was not afraid of directors of Opera Houses.

'People say that you're a friend of the Opera ghost, Madame Zwingli,' Mr Arthur began. 'Tell us about him. Some people say he has no head.'

'And some people say he has no body,' said Monsieur Francis. 'What do you say, Madame Zwingli?'

Madame Zwingli looked at the two men and laughed. 'I say that the directors of the Opera House are fools!'

'What!' Mr Arthur stood up. Normally, he wouldn't be this impolite. But all these ghost rumours really stressed him out. 'Listen, Madame-'

'Oh, sit down, Arthur, and listen,' said Monsieur Francis. 'Why do you say that, Madame Zwingli?'

'Because, Monsieur, the Opera ghost is angry with you. When the ghost wants something, he must have it. He is clever and dangerous, this ghost. The old directors before you, they knew that, oh yes. At first they tried to stop him. Then there were many accidents in the Opera House, many strange accidents. And when did these accidents happen? When the ghost was angry! So, the old directors learnt very quickly. The ghost wants Box 5? He can have it every night. The ghost wants money? Let's give the money to him at once. Oh yes, the old directors understood very well.'

'But we are the directors, not the Opera ghost.' Mr Arthur was impatient. He turned to Monsieur Francis. 'This is outrageous. Why should we listen to this lady? On Friday night Isabella Fernández Carriedo is going to sing Margarita. And you and I, Francis, are going to watch the opera from Box 5.'

'Well, we can try that, Arthur. But we don't want any accidents.'

Madame Zwingli came nearer to the two men. 'Listen to me,' she said quietly. 'Remember Joseph Buquet? I tell you, the Opera ghost is a good friend, but a bad enemy.'

The two men stared at her. 'Those words,' Monsieur Francis said slowly, 'why did you say those words, Madame Zwingli?'

'Because the ghost says them to me. I never see him, but I often hear him. He has a very nice voice.'