The comic that bridges the story between Portal and Portal 2 is Lab Rat, officially titled Portal 2: Lab Rat. The digital comic follows Doug Rattmann, the Aperture employee known as the Rat Man, and fills in crucial events omitted from the games.
Lab Rat explains how Rattmann survived GLaDOS’s neurotoxin attack, recognized Chell’s unusual tenacity, and moved her to the front of the testing queue. His graffiti and hidden wall drawings in Portal are given a direct story purpose: he is trying to guide and warn Chell from inside the facility.
After Chell defeats GLaDOS, the comic shows the Party Escort Bot dragging her back into Aperture. Rattmann returns, helps place Chell into long-term stasis, and is wounded by a turret. He then enters a stasis pod himself, leaving his later fate unknown.
Valve released the 27-page comic online in two parts shortly before Portal 2. It was later included in the printed anthology Valve Presents: The Sacrifice and Other Steam-Powered Stories.