(If you know Portal 2's plotline, you can probably guess where this is going. It's just a bit tweaked, since this Wheatley is human.)

 

When Wheatley was hired by Aperture Science, he was over the moon. A rather poor student in high school and college, most people just assumed he wasn't going anywhere. However, depiste his less-than-stellar grades, Wheatley had a passion for Science, and all that came with it. 

 

As one can imagine, things went downhill rather quickly. Wheatley was placed on a team of scientists responsible for the development and creation of GLaDOS. Though he wasn't technically a big part of the AI's development, he certainly thought he was. The unveiling of the massive computer was supposed to be a smooth, impressive ceremony. 

 

Believe it or not, GLaDOS did not play along. in fact, she- it- did quite the opposite, flooding the entire facility with neurotoxin, and killing all the scientists in the area. Well, almost all of them. Luckily for Wheatley (and a few of his collegues), he just happened to be near the cryo-stasis chambers. Those four scientists were able to take shelter in the pods, and survived. 

 

Wheatley was asleep for a very long time. When he woke up, he felt as though no time had passed. His collegues were gone. He was alone. Luck continued to be on his side, however, as he soon ran into Chell. She was in a similar state, having just awoken from a slightly shorter sleep. 

 

Nevertheless, the two paired up. Wheatley provided the knowledge of Aperture and it's interworkings, while Chell provided... Everything else. They were a good team. For a time. 

 

GLaDOS had remained in power during this time. Wheatley and Chell knew that the only way out was to confront her. So, after making it through the scientific hell that was Aperture Labs, that's exactly what they did. 

 

From the get-go, Wheatley knew that things... weren't going to go in their favor. The testing facility still needed something- or someone- in charge, or else the entire system would shut down, and they'd be trapped, forever. It didn't take long for Chell to figure this out, as well. 

 

Lucky for her, she had a scientist on her team. 

 

Managing to override GLaDOS, Chell helped plug Wheatley into the mainframe. Various wires and electrodes were stuck directly into his spinal cord and brain, and if it weren't for the system itself working to keep him alive, Wheatley would have died then and there. 

 

The entirety of a gigantic science facility, plus all of Aperture's plans, blueprints, and secrets were suddenly in his control. Not to mention the absolute insanity of having an AI's thinking power inside a human's brain. 

 

And Wheatley did, in fact, go insane. All that power, given to a lowly scientist who'd previously had none? Wheatley turned on Chell like a feral animal, shoving her back into the testing areas they'd just escaped from. Addicted to testing in a way that no AI would be, he nearly maimed or even killed his former friend on multiple occasions. 

 

Chell and GLaDOS were forced to team up. Once again, with the AI's knowledge of Aperture, and Chell's... everything else, they were able to take Wheatley down, and end his reign. 

 

Chell managed to escape. Wheatley was not so lucky. Unfortunately for him, they'd apparently programmed a sense of revenge into the AI. 

 

He still had family and friends back home, however, so GLaDOS made up a story that he'd gone to space. A truly unbelievable one, but they bought it anyway. Wheatley remained on Earth. In the testing labs. Alone. With the AI he'd taken down, mutilated, and attempted to destroy. 

 

Wheatley is out of Aperture Labs now, but... he doesn't like to talk about those long months very much.