Millie doesn’t even buy sunscreen that isn’t reef-friendly. Any marine biologist worth their salt wouldn’t dare do that. Unless I can find the right kind of sunscreen before I get on the boat, everyone will know I’m a fraud, let alone the guilt I’ll feel using a product that could damage the environment I’m supposed to have dedicated my life to studying and preserving. I’m supposed to board a shuttle to the pier as soon as I land in Cairns, and there is zero chance that I’ll be able to find reef-friendly sunscreen at Dallas airport.

I resist the urge to smack myself in my forehead with the heel of my hand.

‘Are you OK?’ the flight attendant asks as I scooch by her onto the plane.

I gulp. ‘Mhm,’ I say, hoping she thinks I’m scared of flying and doesn’t register that I’m about to have a full-on mental breakdown.

What was I thinking? I ask myself as I sink into my seat. I’m in the window, squeezed next to a man with a giant Kindle on his lap. He is paging through a romance novel in exceptionally large font, something I would usually find amusing, but I’m so stressed that it doesn’t bring a smile to my face.

How am I going to convince people that I’m Millie if I can’t even remember the basics? Why did I think this would be a piece of cake? I begin to regret spending more time studying the coral and fish than reviewing scuba basics. Millie kept telling me, ‘It’s just like riding a bike,’ but it’s been almost five years since my last dive. Not to mention I’ll be on a boat with total strangers, and I’m travelling all the way there and back alone. I’ll have no one to stop me from making a complete fool of myself. No, worse, a complete fool of my sister.

I close my eyes to keep from crying. If Millie thinks I can do this . . . then I can do it . . . right? As the plane takes off, a familiar thought hits me, one I’ve had a thousand times before: I wish I was her.

I ease her scuba certification card out of my wallet. The photo is small and grainy. I can pass for her easily, the same way I did for those eleven months when she was twenty-one and I used her ID to get into bars.

I only indulge myself for a moment before a shake of turbulence brings me back to my senses. How dare I wish I was Millie, how could I have the indecency to want to trade places with her, when I am jetting off to Australia and she is getting ready for what will probably be the worst week of her life?