They inject you with something and your body starts to change.

A hatch in the door opens up and a metal tray slides through, a single syringe on it.
The Doc picks it up and turns to you, a devilish gleam in his eye. "Now, Mr. Redding, try not to move around too much. You would want me to miss..."
You try to move away as he brings the needle to your neck but your bindings are far too tight for that to make much difference. You desperately try to scream as the sharp metal slides into your carotid artery, the strange sludge pushing its way into your body. As the last of the stuff enters your body, the spot on your neck starts to feel like it's burning. It starts to spread throughout your body, making you feel like you're on fire. Soon, it reaches your brain and your whole body seizes up, your every muscle made tense.

But then, your body starts to change. It starts in your bones. You feel them start to shift, seemingly having a mind of their own. They're growing, shrinking, some seem to pop into place as your entire body shifts into it's new form. Your feet shrink to a third of their original size, your hips expand to twice their size and your gut seems to collapse into itself (leaving you in the best shape you've ever been.) The fat and muscle in your arms and legs seem to drain away. Then, your chest starts to expand. They start to balloon outwards from your nipples, unmistakingly becoming a pair of breasts. They fall away from you as gravity takes hold of them, giving you a perfect view of the final step of this transformation.
All of the energy in your body begins to pool together in one place; your cock. Still hard up from Jake's earlier ministrations, your dick seems to throb harder and harder every second. You have never felt the need to cum this badly in your life!
Behind you, Jake says to the Doc "Turn her voice back on! I want to hear it happen!"
Suddenly, you find your voice again, a hoarse scream ripping out of your mouth. It's at a much higher pitch than you've ever heard it, but it's definitely you. "AHHHHHHHHH!!!"
With a screech like the girl at the end of a horror movie, the tension in your body finally breaks. You cum the hardest you've ever cummed in your life. Hot seed shoots out of your dick, spraying all over yourself, some even landing on your face. As it pools between your new breasts, your cock keeps spasming, the burst of pleasure fading fast as it begins to shrink. Soon, you feel like your cock is yanked inside out.
It's gone. All you feel down there is an absence. You don't want to think about what's there now. But you can't help it. You are now a woman.

The ordeal over, outside senses begin to return.
The incessant beeping of the heart monitor. The harsh lights above you, stinging your eyes. The chill on your skin. But all of that doesn't seem that important anymore.