You lay in bed and wonder how long you should wait before taking the tail off. You realize you're going to need to clean it now before you put it back. Before you do anything you hear the front door open and close. Your girlfriend must be home! You're going to get caught red-handed now unless you act fast. Maybe if you get the tail off quickly and hide it you could clean it and put it back where you found it later without her noticing? You try to find the seam around your waist to pull the tail down, but you can't! You're still struggling with the tail when your girlfriend walks into the bedroom and freezes in the doorway at the sight of you.
"You found my tail!" she exclaims.
"I'm sorry - I didn't mean to ruin your surprise. I was putting away some laundry and found it, and I let my curiosity get the better of me. Can you help me get out of it?"
She gives you a curious look and walks toward the bed. "You can't find the seam? How long have you had it on?"
"I dunno, maybe 10 or 15 minutes."
"It should take longer than that...unless..." Her eyes light up. "You masturbated while wearing the tail, didn't you?"
You're surprised by the tone of her voice - more curious than angry. "Yeah..."
"So you like being a merman? It turns you on?"
"Yeah, although not as much as imagining you as a mermaid does."
That was clearly the answer she wanted to hear as a huge smile spreads across her face. She climbs onto the bed and gives you a huge, passionate kiss.
"I've got good news for you then..."
"You've got a tail for yourself?"
"In a manner of speaking. I'm a mermaid, but I can take human form."
"So this tail was always for me to wear?"
"Yep, for the day you were ready to join me."
Kneeling on the bed, straddling your tail, your girlfriend closes her eyes clearly concentrating on something. After a few moments her whole body visibly shudders and she lets out a sigh.
"My transformation has started." She gives you a sly grin. "I better get out of these clothes..."
Still straddling your tail, she slowly and seductively unbuttons her blouse then casts it aside, followed by her bra. Her breasts sway and she fondles them, putting on a show for you for several minutes before removing her skirt. You can see the colorful scales spreading across her legs. She begins to grind against you until her transformation shreds her pink panties, revealing her wet slit amidst a sea of scales. As her legs begin to connect as if by a zipper moving down from her crotch she can no longer straddle you so she flops over onto the bed next to you. You watch as the changes progress down her legs and her feet flatten out into a tail fin, completing your girlfriend's transformation into a mermaid.