Late evening, on a rooftop somewhere in the city...
"It's Elastra! Run!"
Out of the darkness, a fist stretched out to inhuman lengths in a flash, smashing into one of the fleeing criminals. The other two looked in shock and took off across the rooftop.
A leg flew out, tripping a second black-clad villain, sending him flying. A flying, outstretched fist quickly rendered him as unconscious as the first. The fist unstretched back to a white-costumed heroine.
"One left." Elastra spoke, reading to pounce. "Give up now! There's nowhere to go!"
The final criminal looked panicked, back facing a ledge and a very long drop to the street below. He reached into his coat for something, but Elastra was faster, rushing towards him by stretching her torso and knocking him to the ground and out cold.
"Hah! That was so easy!"
She jumped up and down, unable to contain her excitement, but quickly collected herself. Reaching down, she grunted and lifted the man she'd just knocked out onto her shoulder. As she did so, an object fell out of his hand and hit the ground with a clink.
"Huh? What's-" Elastra had no time to finish the thought as a flash of light and a cloud of green gas enveloped her and all around her. "W-what is... uh..."
Elastra's vision went hazy. She tried to stretch away, but her muscles refused to respond, becoming weak and heavy. She took a step back and her foot missed the ledge. Her last conscious view was of a swiftly approaching dumpster. And then darkness.
A moment later a door opened and a young, scrawny teen stepped out of the store with an armful of comic books and looked towards the alley.
"What was that noise?" He asked.
Cautiously, he took a few steps down the alleyway and glanced into the dumpster.
"Holy shit!" He said, dropping the comics and grabbing Elastra, pulling her out. Her body had deflated slightly in her unconscious state.
"Did the comic shop throw this out? I've never seen such a detailed bodysuit!"
Glancing around to make sure he was alone, he did what any comic nerd in his position would do, and quickly absconded with the "costume" he'd just acquired, comics left forgotten.
A few minutes later he was back at his apartment, admiring his lucky find. He'd laid her out onto his bed, arms and legs outstretched.
"It feels so realistic." Kevin whispered as he unknowingly fondled the body of Elastra in his hands. "Her costume is completely authentic and I swear the skin feels like real skin!"
He brushed his hands over her face, admiring the detail that must have gone into making a costume so perfect.
"I've got to try it on!" He said.
It was at that point that he realized there was an issue: how did one put it on? He looked her body over, searching in vain for a zipper, clasp, button, latch... anything that would allow access into his amazing new costume, but to no avail.
"There's gotta be some way in this thing! Who would design a costume without an entrance?" Kevin muttered.
Running out of options, Kevin found himself looking to the only naturally entrance he could think of: her mouth. (Well, there was another, but he didn't want to consider that one yet)
Experimentally, he pried at her mouth, and was amazed to see it stretch open like... well, exactly like how Elastra's powers allowed her to stretch. Clearly, this must be how the costume is worn.
Excitedly, Kevin began to undress, pulling off his shirt, kicking off his pants, and finally shimmying out of his boxers. Not wasting any time, the pail, wiry teen grabbed the costume and shoved one leg down the throat then the other, watching it easily stretch to accommodate it.
With some effort, he pulled Elastra's body up his own. It felt a little weird, climbing into her this way. He had to adjust his dick as it reached her mouth, shoving it down inside so he could keep pulling it on. His legs slid down her torso, stretching past her abdomen, and then into her own legs, filling them out like stockings.
Finally he began stuffing his arms into hers one at a time, before using her hands to grab the "costume's" head and shoving it down onto his own.
The process was complete. He was now completely inside the superhero's body, and it was time to admire the results.
Meanwhile, the heroine began to slowly stir. Her head was still ringing and foggy from the fight, and she tried to remember what had happened. She felt odd - stuffed in a way that didn't feel natural. And it almost felt like she was moving. Slowly she opened her eyes.
As the world swam back into focus, she found herself looking... at her own body. Or rather, a reflection in a mirror. Trying to figure out where she was, she attempted to look around, but quickly realized she could only move her eyes - her head refused to move along with them. She tried to move her arms or legs, but found she couldn't do that either.
Then, to her shock, her hands began to move on their own, exploring up and down her own body, touching herself. When she tried to stop, nothing happened!
'What trick is this?!' Elastra cried out in her mind.
"This is incredible!" Elastra suddenly said, her mouth and throat sounding out the words against her will. "Wow, it even has a voice changer too? This costume is amazing!"
'Costume?!' Elastra thought. 'Is... is someone inside me?!'