Kevin let out occasional moans of pleasure as Lilia continued to pleasure her boyfriend-turned-mermaid. Lilia straddled Kevin's tail which instinctively flapped against the arm of the couch. The more Lilia's tongue drove Kevin to new heights of pleasure, the more she wanted Kevin to return the favor on her. Finally Lilia could wait no longer and sat up on the couch.
"Wow..." said Kevin, feeling at a loss for words. "That was...wow..."
"Ready to the return the favor?" asked Lilia.
"Let me just take...a moment...to catch my breath..."
"Please..." pleaded Lilia "I need you."
"How can I say no to that?"
"It's not just being horny. I want my tail, dammit. I want my tail right now like I've never wanted anything in my life. My legs are screaming, but the serum prevents me from changing. I don't know if an orgasm would trigger a change any, but if I can't be a mermaid getting eaten by one is the next best thing right now."
Kevin required no more prompting. Kevin rolled over onto his stomach and Lilia moved to the other end of the couch, her back against the couch arm and her legs wide. Kevin kissed his way down Lilia's inner thighs and teased Lilia's lower lips before slipping his tongue into her moist slit.
"Yes" moaned Lilia. Kevin did his best, guided by occasional moaned instructions from Lilia. Amid the pleasure from Kevin, the desire for her tail continued to built, yet seemed unattainable. Lilia soon began to caress her own breasts, but was soon lost in pleasure and desire. She was snapped back to reality as she felt a familiar squirming feeling in her legs. Was her change really beginning? Yes, the feel was unmistakable! Waves of pleasure began sweeping over her body like waves on a beach and Lilia instinctively moved her legs together.
Noticing this, Kevin stopped to ask "Is it starting?"
"Yes! I think it's going to work! Keep going!" she screamed with delight.
Kevin resumed his efforts, probing deeper with his tongue. Lilia could feel the change building. Had she looked down, she would have seen scales beginning to appear on her legs, but her head was tilted back and her eyes closed. She began moaning with pleasure. Finally the orgasm hit like a tsunami and the remain changes swept down her legs to the tips of what had been her toes and were now her fins.
It was Lilia's turn to lay there panting and struggling for breath. It had worked - she had her tail. Kevin ran his hands over Lilia's tail, still amazed by it all. The two exhausted and naked mermaids lay on the couch in each other's arms.