going back a bit you took notice of this mermaid costume (sapphire tail, green bivalves shell bikini top, butt length brunette wig) that just...felt like the perfect option. curiously a note on the hanger holding the tail said 'one size fits all: guaranteed', "can... can that actually be true?" you ask yourself as you find your hand edging towards grabbing the costume.
thankfully you did remember to bring your wallet (not purse due to your tomboyish 'phrase' as your mother called it) with a middle three digit amount in it and failing your emergency credit card in the event you inadvertently ruined the costume trying it on, so once the costume was in your hands you quickly hurried off to the changing rooms.
once inside you hung the costume onto the door's hook after locking it before stripping off your tatty jeans and sneakers before sitting down to remove your sleeveless jacket and t-shirt...
...once down on the included bench in your underwear you briefly got back up to collect the costume. first getting the wing to place it over your own neck-length hair (natural brown/dyed or another colour of reader's choice of view), "oh... that tickles a little" you self-joke as you find how easily the wing adapts to your head.
next you remove your bra due to how the seashell bra was designed not to accompany undergarments and replaced it over your C-cup breasts with the new shell bikini top and actually easily managed to clip it on and noticed how comfy it felt on.
"and last but most certainly not least" you announce as you bury your feet into the tail after rolling it up slightly to see where to put your feet, easily finding to cup corresponding to the monofin if the tail. once your feet were in place you got up and started rolling the tight tail up your legs now being locked together inside.
the process of getting the tail up over your legs proved to be the longest aspect of getting this costume on. yet despite how hugging the material was inside, outside no matter how high the tail went you saw no sign of your legs underneath in the room's mirror and towards the end as the tail now covered both your ass and panties you felt like you needed to sit down to make the final adjustment.
"oh... hello well toned mermaid in the mirror, where- oh wait, your me" you joked with the reflection as you leaned back slightly to admire the results that proved this costume was one size fits all like the note said...
...but suddenly everything started feeling off, first with the slight breeze starting to be felt on your head through your hair that was suppose to be covered by the wig, making a grab to check you were taken back at the feeling of your hand tugging at your own natural hair.
"what the f-" you almost cursed before the feelings in your lower body started shifting. first with what felt like your panties fading out of existence within the tail before the skin of your legs started merging your legs into a single limb, the muscles following suite.
then your feet combined together with the monofin to create a real tailfin as the material of the tail costume started to meld together with your skin below and morph into real, genuine scales from your waist right down to your new tail fin...
...the final part being your breasts expanding from C to D-cup, finishing your process from human to mermaid with all your bones below your stomach having shifted shape.
"bloody hell this isn't happening?!" you cry as your fanatically start pulling at where the seem of the tail had once started but only found flawless transition from skin to scales. panicking, you push yourself down to the floor and struggle your way through pulls to the door in the hope of finding help: