All of a sudden Zack seemed to shoot back through time, losing a few years until he reached 13 and saw himself in his room, posing in his underwear in front of the mirror. He shivered and felt a strange tingling sensation start to run through him, and went to lock his door, returning to the mirror and looking at himself confusedly.
Instead of passing like he expected it would, the tingling seemed to grow more intense. It felt strongest in his crotch, where he had an erection, and weirdly, in his chest. He had never really thought about his own chest before, but he felt like he wanted to touch it.
The desire in his body was instinctive and insatiable. He felt his nipples perk up and get erect, and like when his penis got like this, he felt a huge urge to play with them. Gripping one between his index finger and thumb, he gasped at the sensation.
His other hand moved to stroke the other, and he started to gently rub. Under his touch, the nipples grew and grew, and soft mounds of flesh poked out of his chest under them like two erections. This felt amazing, but he also felt confused- what was this feeling? His body felt so odd, and it looked so different all of a sudden, but he didn't want it to stop. He felt like if he kept going, he would understand this pleasure.
As he looked down to his crotch at the mirror and saw the contrast between his feminine chest and masculine crotch, he realised what this was. He'd started to change from male to female.
In response to his acknowledgement, his penis and testicles moved up inside him rapidly. His shaft felt just as hard, but it shrank down to a tiny size, much too small to fuck someone with but much more sensitive with how its nerve endings were bound so tightly together. His reproductive system totally reworked itself- testicles became ovaries, semen turned to vaginal juices, and a vaginal tract and womb formed. His scrotum receded from a protruding sac to a nearly flat mound under his newformed clitoris, and then the seam down its middle unsealed to form a vaginal slit.
While his genitalia reshaped, Zack had been through a rollercoaster of feelings. For a moment he had felt fearful and uncomfortable about relinquishing his manhood, but as the change had gone on, not only had he enjoyed it, she managed to accept her new gender. It felt so nice to be a girl, she didn't even want to object to it. She even settled on a name she'd like to be called- Zoe.
Once she came down from the high of her transformation, Zoe felt aware of her own nudity and blushed hard, covering her chest and crotch with one hand. She threw on a T-shirt, underwear and some jeans, tightening up the belt big time, then made her way to the bedroom door...