I jump to my feet, pushing my cap out of my eyes, but Boathook Man is between me and the window now, bringing his hook hand up again. I grab something from the nearest crate – an old silver tea tray – and hold it like a shield just as the hook comes crashing down. I see the underside of the tray bulge and split as the hook punctures it. The tray is wrenched from my hand.

The man bellows with fury. I’m running now, straight for the window, but I don’t think I’ll make it.

Then something white flashes past me and flies up at Boathook Man. It spits and hisses and wraps itself around the man’s face. I look back and realize what it is.

“Erwin!”

The cat is attacking the old mariner’s head ferociously, raking at him with his claws. But instead of blood, only water spouts from the wounds.

“Come on!” shouts Vi, and I don’t need telling again. I grab her hand and she pulls me out onto the roof. Or, rather, the narrow stretch of roof below the windowsill. The snowy ground four storeys below wavers before my eyes.

The Boathook Man finally gets his hand on Erwin, and the poor cat is flung to one side.

I wish I could do something to help Erwin, but it’s all I can do to edge away from the window before Boathook Man lunges at it, his face a blank roar, his beard streaming with salt water from his torn face. He thrusts his hook hand forward again but the tray is still stuck to it and it catches on the window.

“Don’t look down!” says Vi, pulling me further along, her feet braced against the lead guttering. It bulges with our combined weight. The roof is steep, and the tiles offer no grip whatsoever.

“Where should I look, then?” I gasp, as the guttering makes another groan. “Back?”

I give “back” a try and instantly regret it – Boathook Man is already climbing out of the window, the tray shaken free of his hook.

“The next-door roof looks flatter,” says Vi. “And it’s only a small jump.”

“Jump?” I say.

But what choice do we have?

Boathook Man is standing on the guttering now, which shrieks and twists as the metal gives under his watery bulk. And I’m not far enough away. He raises his hook hand to strike at me yet again, so I duck…

Then something happens that changes everything.

An object flashes over my head.

It’s one of Sebastian Eels’ steel harpoons.

If I hadn’t been ducking at that moment, the harpoon wouldn’t have been flashing over anything, because it would have been going straight through me!

Instead, it strikes the roof tiles above with a crack, sending up an explosion of snow, before ricocheting back …

… and clonking Boathook Man in the face.