Roxy has turned off “caller ID,” which she says is a way that the recipient of a call can identify you.

She has given a false name and (presumably) a false number to whoever answered her emergency call, and then wails, “It’s me dad! He’s in trouble! He’s on a windsurfer and he’s miles out to sea!”

She is good. Very convincing. I look at her face and it is contorted with worry, as though she really is making an emergency call about her dad.

“He went in off Low Hauxley Beach,” she says. “You’ve got to help! Thank you….Yes, I’ll stay here.”

We do not have to wait long. Within a few minutes, three figures in yellow oilskins run out of the Coast Guard building and climb down a metal ladder on the harbor wall to a waiting rescue boat: a large, gray, rigid inflatable.

By the time the boat has started, Roxy, Aidan, and I are down by the marina. I am breathing so hard with nerves that it is making me a little light-headed.

“Will you hurry, please?” says Roxy into the telephone. “I’ve got no battery left on my phone. You are comin’?…Thanks, I’ve gotta go ’cause—” And she shuts the telephone off mid-sentence.

“Are you OK?” asks Aidan, but I can only nod in response.

He turns to Roxy. “All right. Time to do your thing.” She tightens her jaw and marches behind a shuttered ice-cream shed, emerging seconds later in a T-shirt and underwear.

There are people on the jetties—yachties, mainly—but no one seems to take much notice of us. Not yet, anyway.

“Jolly good luck, Roxy,” I say, and I offer her my hand to shake. Her face breaks into a grin.

“You, Alfie Monk, have to be the weirdest kid ever!” she says. “Come here!” She throws her tiny arms round me and squeezes hard. “You are the one that needs good luck. All I have to do is pretend to be drowning. Jolly good luck. Ha!”

With that, she descends some steps from the jetty into the water and slips in without even gasping at what must be very cold water. She is clenching her jaw so tightly that thick sinewy cords run down her neck, and her eyes are so white and wide open that I fear they may pop out.

I tell you: Roxy Minto is the toughest girl I have ever met.

She begins her swim out into the middle of the harbor, and nobody even notices her tiny dark head bobbing through the water.