‘We’ll get worse than hurt if we stay around here.’ Ryan put two of the deadly marbles back in the little cylinder, holding the third in his palm. ‘And if we go now we’ve got the element of surprise, haven’t we?’

‘I’ll be surprised if this actually works without blowing us to bits,’ said Graham, looking at the marble in his palm. He gave Ryan an encouraging smile. ‘Still. No time like now, eh?’

The two of them backed away into a corner, as far from the door as they could get. Graham had an old tissue in his pocket and made slightly grotty earplugs to protect them both against the blast. Then he took a deep breath, and threw the Nitro-90 at the hidden door in the wall.

It struck the surface and bounced back at them.

Graham swore. ‘The wall’s proper gold. Gold’s not magnetic!’

Ryan kicked wildly at the ball and caught it a glancing blow, just enough to send it skittering back towards the wall. Then he and Graham turned their backs – as the tiny explosive went off.

The noise was insane, the heat was instant and fierce, and Graham feared it had burned the clothes from his back and half his skin with it. The shockwave flattened him against the wall he was braced against, and an alarm began to howl. They turned through the smoke to find a gaping hole in the floor and wall, strange components within spitting sparks from the darkness.

‘Come on!’ Graham yelled, ducking down under the glittering, jagged remains of the doorway.

Ryan was right behind him, a Nitro-90 sphere in his fist, staring around. ‘Where do we make for?’

‘Why are there no signs in this thing!’ Graham said despairingly.

An urgent clatter of hooves suggested company was on the way.

‘Well, it was that way to the think-scanner –’ Ryan pointed down the corridor – ‘so let’s try the other way.’

A blast of brilliant energy flashed over Graham’s head and tore sparks from the wall behind him. ‘Let’s try it right now!’ he shouted, leading the way along the high arched corridor.

Ryan yelled back over his shoulder: ‘Stay back, we don’t want to hurt anyone!’ He threw down another marble. The blast seemed to follow almost at once, knocking both men off their feet.

Graham looked back dizzily and saw a massive hole had been ripped in the floor, and a section of the wall had caved in. Another alarm, higher pitched, added to the cacophony.

A centaurion loomed up through the smoke, raised its golden crossbow. Graham stared death in the long face.

Then a ceiling panel fell onto the centaurion’s head, knocking it cold.

‘Oh my days,’ Ryan said, ‘now they’re really gonna be mad.’