I think about saying no. I think about explaining that sneaking Violet up to the fourth floor without being seen is impossible, not with the chambermaids on their rounds and Mr Mollusc on the prowl. But one look at Violet is enough to tell me there’s no point.

Somehow or other, we’re going to Room 407.

Five minutes later, I’m lurking in my cubbyhole, waiting for the coast to clear.

The Grand Nautilus Hotel is a strange place. In high season, it’s full of summer guests – people in shorts and shades and sunburns who drift about looking at everything and noticing nothing.

But it’s not like that in the winter. In the winter, people only stay here if they have a good reason. Or a bad one. These are the kind of people who look at their reflections in the lobby mirrors just to see if they are being followed. Or ask to be seated where they can see but not be seen. These are people with secrets to keep or secrets to uncover. These are people who notice everything.

And that’s why I’ve waited till there’s no one at all in the lobby except me and Amber Griss.

The hotel receptionist is standing as usual at her enormous mahogany desk. As I saunter over to her – all casual and nothing’s-up-at-all – she makes a face at me that says, “I like you, Herbie Lemon, I always have, but I’m tired of defending you from Mr Mollusc, so don’t try any funny business with me today, OK?”

I have a bit of a reputation.

“Morning, Amber,” I say. “There’s some funny business going on outside. I think you should go and see for yourself.”

“Now, Herbie…” Amber warns, peering over her severe spectacles.

“No, really!” I say. “Did you hear that someone stole some croissants from the kitchen this morning?”

“Yes, I did. Chef is furious —”

“Well,” I say, “the thief is just outside, selling those croissants round the side of the hotel!”

“What?”

“I can’t do anything about it because I’m only twelvish, but someone needs to before old Mollusc finds out.”

“Old Moll—” Amber starts to say. “I mean, Mr Mollusc will go ballistic if he finds out. But I can’t just leave Reception.”

I tug the front of my uniform straight and snap to smart attention.

“I’ll cover the desk for a moment,” I say. “You can count on me.”

Amber looks uncertain.

So I try to look fresh-faced and dependable.

“Fine,” Amber says eventually. “Thanks, Herbie. I’ll be as quick as I can.”

And she leaves her desk and clip-clops across the marble to the great revolving doors, leaving me feeling triumphant and clever and, yes, also a bit guilty.

But I can worry about that later.

I nip round behind the Reception desk, and glance both ways.

Now I’m completely alone.