“What? Have I just licked a wall with arsenic in it? Why did you make me do that? Ugh, I feel sick.”

“No, it’s all right, it really is, I promise, I read a book about it. Look, I’ll lick it too if you like.”

“Why would I want you to lick a cave wall? Now what are you doing?”

“I’m going through that pool to see what’s on the other side.” Finn rolled his jeans up above his knees and waded into the clean blue stillness of the cave-pool. “Man, that’s cold.”

“Are you sure it’s safe?”

“It’s probably all right.”

“Probably?”

“Well, everyone does it but you can never be totally sure in a cave.”

“Finn,” said Caroline, “come back. Right now.”

“It’s fine.”

“Finn,” Caroline repeated, her voice trembling slightly, “please, come back outside. I’m cold and I’m frightened and I want to go back to the beach and enjoy the sunshine.”

“What? Oh no, you are as well. I’m sorry, I didn’t realise.” Finn splashed back through the pool and put his arm around Caroline’s shoulders. “All right, let’s go.”

“But you haven’t finished.”

“I’ll come back another day.”

“You’d rather be poking around in a smelly old cave by yourself than relaxing with me?”

“They’re not smelly. They’re actually very clean. They get a good washout every high tide and the rainwater drains through them.” Caroline looked at him. “I think they’re interesting, that’s all. But you don’t have to come back with me.”

“You’ll get lonely.”

“No, I won’t, I’ll just bring… ”

“Ava,” said Caroline.

Finn looked confused. “Why is that a problem?”

“Because… !” Caroline threw up her arms. “Because you’d rather spend time with your older sister messing around in some poxy bloody cave than sunbathing with your girlfriend! Normal people come to the beach to get a suntan!”

“You can get a suntan. I don’t mind.”

“But I mind. I feel like I’m competing with Ava all the time.”

“That’s ridiculous,” said Finn. “She’s just my sister.”

“Exactly! Just your sister! I don’t know why you even wanted to come on holiday with her and Paul. He doesn’t want to be here either.”

“Yes, I know,” said Finn. “That’s sort of why I asked them. I think he wants to marry her. So I thought I’d bring them both down here and see what happens.”

“You’re trying to split them up?”

“I’m not trying to do anything. I just thought it would be an interesting experiment, that’s all.”

“Look,” said Caroline. “How is it even your problem? She’s a grown woman. She can make up her own mind. I mean, I know she looked after you when… ”

Finn took her hand. “Let’s go back to the beach.”

“No, we should talk about this.”

“I’m sorry, you’re right, I’m a terrible boyfriend.”

“No, you’re not. I just wish you wouldn’t keep changing the subject whenever I mention… ”

“I’m lucky you put up with me, I really am.” He tucked her hair carefully behind her ears. “So lucky. What would you like to do?”