Penn and Thea were silent for a time, letting Gemma absorb it all. But it was fairly obvious what the story was about.

“You’re the three sisters?” Gemma pointed to them one by one. “Peisinoe, Thelxiepeia, and Aglaope.”

“Not exactly.” Penn shook her head. “It’s true that I was once Peisinoe, and Thea was once Thelxiepeia. But Lexi is a replacement for Ligeia, who died many, many years ago.”

“Wait. Lexi replaced one of you?” Gemma asked. “Why do you have replacements? And where’s your other sister, Aglaope?”

“It’s part of Demeter’s curse,” Thea answered. “We chose our friend and sisters over her daughter, then we must always be with our friends and sisters. There must always be four of us, together. We can never leave or be apart for more than a few weeks at a time.”

“If one of us leaves, she will die, and we have to replace her,” Penn explained. “We only have until the current moon is full to fill her role.”

“I’m Aglaope’s replacement.” Gemma swallowed hard as the realization struck her. “What if I don’t want to be?”

“You have no choice. You are already a siren. If you try to leave instead of joining us, you will die, and we will simply replace you.”

“How?” Gemma asked. “How did I turn? That flask?”

“Yes. It was a … mixture of a few things.” Penn chose her words carefully.

“A mixture of what?” Gemma asked.

Penn shook her head. “It’s nothing to concern yourself with now. You wouldn’t even understand what they were. In time, it’ll all be explained to you.”

“Why?” Gemma asked, a new tremor to her voice. “Why me? Why did you want me to join you?”

“Isn’t it obvious?” Penn asked. “You’re beautiful, you love the water, and you’re fearless. Aglaope was too afraid, and we needed someone different.”

“She wasn’t afraid,” Thea countered. “She was considerate.”

“It doesn’t matter what she was,” Penn said sharply. “She’s gone, and we have Gemma now.”

“So … you expect me to just join you, leave everything I’ve ever known, and spend my life singing and swimming?” Gemma asked.

“That doesn’t sound so terrible, does it?” Penn asked.

“It really is wonderful,” Lexi chimed in. “Once you get used to it. It’s a million times better than anything a mortal life could give you.”

“But what if I…” Gemma trailed off and lowered her eyes, thinking of Harper, her parents, Alex. She lifted her head, meeting Penn’s eyes. “I don’t want this.”

“Then you will die,” Penn said. She shrugged as if she couldn’t care less, but her voice was hard and her eyes burned. “If that’s what you wish, then so be it.”