As Darren held Sierra in his arms she began to bleed out. He began to sob as he berated himself. "I knew this would happen. We should have never agreed to this." He sobbed "Why didn't you listen to me!?" He shouted as Sierra consoled "Don't worry. I'll be alright but I want you to remember: no matter where I am I will always lov-"Wait, wait, wait, wait. I'm getting ahead of myself. Let me start at the beginning.
During to GLaDOS' takeover of the enrichment center one engineer Doug Rattman (A.K.A The rat man) survived and continually evaded GLaDOS until Chell's defeat of GLaDOS. In the final explosion Chell was knocked unconscious and dragged back inside the enrichment centre. But without GLaDOS maintaining the facility the relaxation centre where she was placed was non functional. Without its life support Chell would soon die. So Rattman diverted all the reserve power in the enrichment centre to her life support but in the process was severely injured so he took time to recover. However by the time he healed and was prepared to escape Chell was awakened and was helped to escape by Wheatley but reawakened GLaDOS in the process. So Rattman and Chell were forced to stay in the enrichment centre. After Chell defeated GLaDOS and banished a corrupt Wheatley to space GLaDOS released Chell. But by doing so GLaDOS inadvertently showed Rattman a way out of the enrichment centre, so Rattman took the opportunity to escape and reported the events that transpired in the enrichment centre to the human authorities and counseled out of his schizophrenia.
Despite her incredible power and omnipotence over the centre GLaDOS was defenseless against the power of the Military that invaded the centre and shut her down. The salt mine the enrichment centre was built in was eventually cleared and put up for sale. Doug Rattman, who became a huge success, bought the salt mine and re-opened aperture science, seeing great opportunity in the industry of science. Emulating Cave Johnson's engineering of high-tech science products (most notably the aperture science handheld portal device) and testing them in intense situation test chambers navigated by highly trained athletes.
This brings us to the story of Darren and Sierra. Both were about to lose their apartment because neither held a job. They were then desperate for the money. Sierra was suggested to enter the testing by her mother who also worked in the enrichment center; Darren found the ad in the paper and both were willing to do anything to avoid losing their homes. Darren was an excellent athlete. All through high school he was the captain of the football team and was admitted to MIT on scholarship so when he saw the ad read "Athletes needed." He signed up without a second thought. "I don't know, you're always saying how dangerous your job is and how you should be paid more for this type of risk." Sierra pondered skeptically. "Oh, come on. It'll be fun. Maybe we could sign up for the co-operative testing initiative if you get as good as me." Her mother reassured jokingly. "I don't know."
"Just try it. If you don't love it in a week you can quit."
"Fine."
"Great! I'll get the paperwork started." If either Darren or Sierra knew the tragedies to transpire neither would have ever given the ad a second look