The dying mermaid cryptically implies to Tim and Leah that she doesn't live on actual water, but the depths of the soul.
Tim misunderstands and thinks she drinks blood, but Leah says that's wrong, and in fact Leah is the one who used to drink blood. Now she just starves herself.
Tim is acting nervous around Leah, but it's more because he's attracted to Leah than being scared of her. Leah goes to sleep in the tent. Tim, after being conflicted, sleeps outside.
Leah has her own conflicts, and succumbs to them. She changes form to look like Molly, and makes out with Tim.
Molly, now that she's eaten Faerie food, is converting to being a mybrid human/Faerie, partially due to her often-referenced innately fey nature. Her clothes, and the ground, burn at her touch.
She decides to visit the castle to talk with Titania, and manages to use magic to convince a bridge and a door to let her enter. She burns the axe of a guard and brazenly marches into the court, confronting Titania. The other guests leave in a hurry.
Molly, now surrounded in flames, tells Titania off, explaining how her trick feels like murder, or rape, stealing away her future and her family, and Tim. Titania is shamed, and tries to show her sympathy, but Molly is ticked off, and flares up at her touch.
Tim, after some minutes of exuberant groping, realizes that it's Leah, not Molly. He's angry at her trick, but also feels true attraction to Leah anyway. Leah is touched and tells how she started off as an angel, before being damned by joining Lucifer. Since then, Tim is the only person who cared about her.
They kiss again, and their conjoining brings down a magical rain, created by Tim as an Opener, which starts the desert to flowering. The mermaid is now cured, and sits in their convertible, now full of life's water.