One thing leads to another and you are soon having sex with Joanne, but it's just not as good as it was the night before. You can't help but wish Joanne was a mermaid again. Joanne seems even less satisfied, and you have trouble bringing her to climax.
You spend the day together, but Joanne seems distant. Finally over dinner, with things seemingly no better, you ask her what's wrong.
"You wish I was still a mermaid, don't you?" she says. There's no anger in her voice.
You sigh. "Yes." You feel ashamed to admit it.
Without anther word, Joanne gets up from the table and goes into the bedroom. You get up and follow her. "I'm sorry..." you begin.
Joanne turns to you. "Don't be. I wish I was still a mermaid too!" Her admission catches you by surprise. "The way I felt last night...I've never felt that way before. So sexy. So lustful. So alive. Today, I feel so much less. You were trying so hard to make love to me this morning and I felt so...indifferent."
She held up the potion bottle. "I desperately want to feel like I did last night. Not just again tonight, but forever."
Joanne uncorked the bottle. "Bottoms up!"
"Wait!" You weren't fast enough: Joanne downed the entire contents of the bottle!
"What?" she asked. "I want my change to be permanent."
"It only took a couple drops of that stuff diluted into your drink to transform you!" you explained. "I don't know what the entire bottle will do."
"Oh. Whoops."
Joanne shuddered. "I think it's starting." She hastily removed her shirt and shorts before shuddering again, followed by a moan. "Oh...yes..." She began to caress herself as she fell backwards onto the bed. Her legs began to darken and merge into a dolphin mermaid tail. But unlike last night, the darkening skin extended past per waist, spreading across her body. Joanne writhed on the bed as the changes continued. Her tail seemed longer, her tail flukes bigger. Her vagina lips grew puffy and visibly wet, flanked by pectoral fins that grew from her hips. Her stomach became firm, her darkened skin flawlessly smooth. Her darkening, heaving breasts swelled into DD size, with massive areolas and nipples. The darkening skin continued down her arms and to the tips of her fingers. Her nails disappeared and a thin membrane formed between each finger. The darkening skin ended at the top of her neck, but her raven hair grew twice as long and her lips thickened.
As the changes seemed to finish, Joanne's back arched and she let out a cry of pleasure as she shook with orgasm.
The overdose of the potion had changed Joanne far more than the few drops from last night. Instead of a simple sexy mermaid, she had become a sultry siren - some hyper-sexualized creature out of a pornographer's dreams. And perhaps permanently. You ran your hand along her tail and she squirmed with pleasure at your touch. Every inch of her was an erogenous zone, and she beckoned you.