Origins

Walter gasped as he looked into the mirror, shocked at the visage in front of him - if you'd walked into the inventors vast basement (basically its own annex to the house), you'd have expected the blonde bombshell to be named Bambi or Tiffany, certainly not Walter. But Walter he, or rather she, was. One would see all the lovely traits from a blonde bombshell in Walter - buxom, blonde, busty, leggy, perfect white teeth, (did I mention busty?), and Walter hastened to take notes.

He'd succeeded! Sure, the experiment was about de-ageing, not becoming some sexual hotrod, but still, he'd transformed!

Just as he turned to take notes in the bimbo form, he felt his skin shift, and before his eyes in mere nanoseconds, he was rearranged to Walter.

Well it briefly crushed Walter's spirit to see how quick his transformation disappeared, he eagerly ran to his machine to be transformed again.

This time, however, the second he emerged from the pod, steam filling the basement, he shivered at the cold, his (her?) naked form freezing cold. His nipples hardened and Walter felt himself become more...energetic, more alive. After some experimentation with fingering and exploring his new body, Walter had established a rule that would dominate this form -

To stay in his form, he needed to stay aroused; the longer he stayed aroused, the longer he would stay in the form.

Well he might've been a mad scientist, Walter was a scientist, so he felt comfortable after a few hours of experimenting to emerge from the basement he hid in for so long. Rebecca and his lazy bones son were gone, which meant Walter could, while staying aroused, get some clothing on and explore the world as a 20-year old bombshell.