Rebecca runs away from the convent before she's been there a month and a half.
She's lucky in her timing fetching Catherine; Beth hands her through the window and puts a pillow in the baby blankets to make it look like Rebecca broke in on her own whenever the baby's found missing, and Rebecca makes a run for the harbor. By the time she gets there it's really obvious she doesn't have any milk left after her absence, and Catherine's wailing heartbreakingly with hunger, but Rebecca shouts over her to get passage on a ship for the value of the coins she took from Father's hiding place in the yard, and once she's aboard she can listen and watch for other babies, offer smilingly to trade off nursing them with the other mothers, and if there's five women nursing five babies and only four of them have milk that should work out, shouldn't it?
The trip is long. Catherine grows, Rebecca measures her against her arm, but then before they get where they're get going Catherine gets sick. She just keeps getting sicker. When she won't try to nurse any more Rebecca kind of knows.
Rebecca wakes up in the morning and sees herself all grimy and pathetic-looking on a backdrop of stars, and she wonders if she'd be able to nurse Catherine back to health, if she weren't just pretending - she doesn't know how to do it, she fiddles with her chest until she thinks it feels familiar -
- she blinks away the stars and Catherine already isn't breathing.
She doesn't remember very much of the next few days but more or less comes to with a piece of salt pork in her hand and one of the other women sitting next to her, rocking her still-healthy little boy.
Rebecca eats her salt pork and gets up and goes over to the railing and decides that if jumping in the water makes her not want to die any more she can just turn into a mermaid, now, can't she, and she jumps.
She turns into a mermaid and claws her way back up the side of the ship once she's got her legs back and one of the crew sees her and tells her they'll pay her if she gets wings and warns them if she feels bugs. She says all right. She sits in the crow's nest with beautiful angel wings, white edged in gold, waiting to feel something.
They're not far out from port; she doesn't notice anything before they arrive and she collects her money. She goes out and spends it immediately on a hot dinner and a room and a chance to sing for somebody who bills himself as an agent.
A week later she's been on stage just once and he's coming at her and it's no longer even thrilling in addition to frightening, it's just plain repulsive and dreadful, and she's cleaned herself up enough by this point that she can -
- well, he doesn't chase her.
She goes back to the harbor in her blue-marble Grecian dress, angel wings spread a little, and walks up to the first boat she sees. She gets out on the water again.