A heavy shell spiralled out of the weapon and landed in the centre of the kneeling boys. Some of them stood up in alarm.

The shell exploded and the balcony tumbled from the front of the building, a fiery mass of screaming, charring bodies. It landed with a crash, leaving a gaping hole in the masonry above it.

‘Now,’ Hoff turned to Greeneye as the latter emerged from cover, ‘that’s my idea of fun.’

Anand was tapping along a section of wall in the library. He was worried about his friend. Tim had been missing ever since he’d been summoned to the Head’s office.

In all this trouble, he hadn’t been able to ask about him.

While he was thinking, his hand encountered a soft spot on the spine of a book. It was one of a whole panel of fake spines, a Victorian decorative device long-beloved of captains who would tell their bugs to go and fetch a particular non-existent title. The spine in question was that of Sir Gawain and the Green Knight and the soft spot was circular, and halfway up the binding. Anand pressed it.

Something made a clunk and then a grind, and the whole panel of books swung open.

The boys cheered and the adults ran over to peer down a flight of dank little stone steps. ‘Open sesame,’ said Anand.

Serif shot three boys who ran across the end of the corridor he was stalking down, and turned the corner a moment later.

He was in the central hallway where the bursar’s office met the foot of the great stairways.

A group of boys looked up from the sandbags they had piled against the great doors, which were secured with heavy bolts. They turned, swinging round their rifles awkwardly.

Serif mowed them all down in a blaze of silent blue light. He broke step only to swing the bodies and the bags away from the doors, which he unbolted and swung open.

‘Come on,’ he called to August and Aphasia. ‘I’m sure I haven’t killed the right boy yet.’

Smith’s party were running down a narrow brick-lined passageway. Smith himself hung back for a moment, noting that Captain Merryweather was always glancing behind him, biting his lip in panic, and tripping over his feet.

Smith drew level with him. ‘What’s wrong?’

‘They’ll come after us. They’ve got all those guns. They’ll kill us.’

‘But we’re cleverer than they are.’

‘That doesn’t matter. That never matters. It’s who’s got the most guns that matters!’

‘Trust me,’ Smith murmured. ‘I won’t let you come to any harm. I promise.’

Merryweather looked up at him uncertainly. ‘Can you promise that?’

‘Of course. You boys may know me as mild-mannered John Smith, history teacher, but secretly I’m the Doctor, universal righter of wrongs and protector of cats.’

‘Are you?’