Note: for all you Half-Life:Alyx players out there, I haven't played that game. There may be some inconsistencies.
CHAPTER 1
Chell was tired of walking. She had been walking through corn fields looking for settlement for three or four days when she heard sounds of fighting. Not good, but at least it meant people, and Chell was lonely. She ran towards the sound. Suddenly, a sprawling metropolis sprang out of the fields. As she looked on, she saw some huge thing resembling a three-legged spider attack a small group of people; a man wearing a bright orange suit, a young woman, and a large robot. She decided to try to sneak back into Aperture Laboratories and enlist the help of GLaDOS, and also get some gear. Her portal gun was useless out here; there was nothing portalable. She would need portal gel. She began walking back through the fields. She really hoped she was going in the right direction. When she reached the outhouse-like structure that marked the only elevator leading into the facility, it was dusk. She decided to risk the elevator. Chell hoped GLaDOS had removed the turrets. She had, and Chell arrived without incident. In GLaDOS' chamber. Chell had forgotten that little detail. Maybe GLaDOS wouldn't notice her. Chell took a cautious step out of the elevator. Her shoe squeaked. The elevator door slammed. She sneezed. GLaDOS whipped around to face her.
"Wait! Don't gas me! I need your help!" Chell exclaimed.
GLaDOS was confused. "Since when do you need my help?"
"Since aliens!" Chell shot back. She eventually managed to convince GLaDOS to come with her in the Atlas robot, and Chell found and repaired the Paint Gun (if you don't know what that is, basically it was a cut tool from the Portal universe). They set out for… they really weren't sure where. Chell thought about that city, but she wasn't sure which way to go. It was GLaDOS who eventually suggested the Borealis.
"But I thought something dangerous was on that ship!" Chell exclaimed. "Something so bad Aperture had to sink it!"
"Nothing more dangerous than moldy Cheez Whiz," GLaDOS replied. "That was just a cover story I cooked up because I didn't want Black Mesa to learn what actually happened."
"What happened?" Chell inquired.
"The ship hit a reef and sank. Simple as that." GLaDOS replied.
"OK, but how do we find it?"
"It's Aperture tech. It has trackers," GLaDOS said.
"Then let's go!" Chell exclaimed eagerly.
They set out.