Surrounded by concrete, it had a low flat roof, asphalt black, on which lay four teenagers smoking cigarettes. Their chests were naked, their t-shirts tied around their waists. There was so much broken glass up there, they could have doubled as fakirs in a magic show. One of them had a pile of sheets of paper, and was folding them into paper planes which he released from the roof. Judging by the number of planes littering the grass, it had been a busy morning at the control tower.

Paint had peeled in long strips from the centre's doors, and one layer of the plywood beneath had been punctured by a foot or a fist. But the doors were locked fast by means of not one but two padlocks. Two more youths sat on the ground, backs against the doors, legs stretched in front of them and crossed at the ankles, for all the world like security guards on a break. Their trainers were in bad repair, their denims patched and torn and patched again. Maybe it was just the fashion. One wore a black t-shirt, the other an unbuttoned denim jacket with no shirt beneath.

'It's shut,' the denim jacket said.

`When does it open?’

'The night. No polis allowed though.’

Rebus smiled. 'I don't think I know you. What's your name?’

The smile back at him was a parody. Black t-shirt grunted an undeveloped laugh. Rebus noticed flecks of white scale in the youth's hair. Neither youth was about to say anything. The teenagers on the roof were standing now, ready to leap in should anything develop.

`Hard men,' said Rebus. He turned and started to walk away. Denim jacket got to his feet and came after him.

'What's up, Mr Polisman?’

Rebus didn't bother looking at the youth, but he stopped walking. `Why should anything be up?’

One of the paper planes, aimed or not, hit him on the leg. He picked it up. On the roof, they were laughing quietly.