You pick the large fish tail costume, which seems big enough to fit one of you in the tail with another sticking out the top, like a mermaid. Or, well, merman.
“Let’s try this one, I’m pretty sure it’s supposed to be a merman. I want to be the top part though,” you say, holding it up to Alex. He shrugs, before replying, “Fine by me,” and starts taking his clothes, prompting you to shout, “Dude, what the fuck!”
“Well, it’s magic so I thought it would work better if we were naked, you should probably do it too,” he explains. You think that’s pretty reasonable, and follow suit by taking your own clothes off, this corner of the store fortunately being clear of any other people.
Once you’re both naked, Alex gets into the costume on the ground like a sleeping bag, with just his head sticking out the top over the v-shaped rim of it. You then squeeze your legs into the tail with your torso outside the suit, and Alex holding your thighs inside the tail as if he was giving you a piggyback, with your crotch pressed against the back of his head.
The two of you lie on the floor in the costume, feeling a big awkward, when Alex says, “So... what now?” That question is answered as you feel the costume suddenly tighten around your lower body, and you both gasp as you watch it constrict tightly your legs and Alex’s entire body. Your skin feels sticky and your legs clay-like as you watch and feel them merge with Alex’s arms and torso inside the tail, fusing your nervous systems and allowing you to share his feeling.
You both groan as Alex’s body, except his head which has now become stuck to your groin, expands and smooths out to perfectly fill the costume, merging with the costume which you can now feel as if its scales were your own skin and leaving you with a perfectly realistic muscular fish tail from your waist down, with Alex’s head on your crotch.
You try to flop your new tail, but are distracted by Alex violently vomiting up... cum? With his head losing its definition and sinking into your abdomen, you feel an unbelievably strong wave of sexual pleasure from Alex’s head, and notice his tongue thickening and protruding from his mouth. It continues to swell into the clear shape of a cock, explaining the source of the cum, and grows to a hefty foot-long, with Alex’s lips being pulled up to form its foreskin, leaving his face unrecognisable.
His last few facial features fade away with his consciousness as you stroke your gigantic new cock, with his chin swelling up to form a scrotum with a considerably hefty pair of balls stashed inside. You quickly climax with an oddly deep moan, the last of Alex being blasted across the store as your thick streams of semen.
You prop yourself up with your arms, and notice something else - your own body has changed massively. You now possess a totally adult torso, with a thick slab of muscle for your pecs, a washboard set of abs, and football-sized biceps, along with a handsome, chiselled, surfer-esque face, combined with long, dark brown hair tied back in a small bun and coating your face in a short beard.
Catching your reflection in the mirror, you see your new form: a 100% authentic merman, with a large emerald-coloured rail to propel you through the water, a huge cock between your scales and torso, and the human body of a model, to which you can’t help but grin widely, despite your inconvenient location on the floor of the costume store.
However, Alex doesn’t seem to be around in your mind or anything like that, seemingly just... turned into your tail and cock and well - now non-existent.