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This article describes the Portal storyline, chapter by chapter.
Portal is set in the Half-Life universe, and is presented to the player through the game's audio messages and visual elements seen throughout the game. Additional elements of the story's background are developed from the fictional Aperture Science website created by Valve for the game.
 Test Chamber 00
Chell wakes up in a so-called Relaxation Vault - with only a small table with a mug, radio, and clipboard listing test chamber hazards, as well as a toilet and a stasis bed of some kind - and is welcomed "again" by a Computerized Voice which states that "her specimen has been processed" and that "we are now ready to begin the test proper."
Throughout the early stages of the game, the voice only refers to "us" or "we," never saying "I," as though it were speaking for the Enrichment Center's long gone employees as a whole. This comes into significance in the later levels, when the entity seems to display more independent activity.
 Behind the scenes
The portal was deliberately introduced in this stage in a way that confirms that they lead to another location on the same planet, rather than another universe, as established in Developer Commentary.
 Test Chamber 01
Here the player reviews how to use buttons, storage cubes, and portals.
 Behind the scenes
This level is designed to firmly establish the mechanics of the Portal Device in the player's mind, so that they would be familiar with the basic concept of the game.
 Test Chamber 02
As the tests progress she eventually acquires the first "portion" of the Aperture Science Handheld Portal Device, allowing her to place blue portals but not orange ones.
 Test Chamber 03
A simple puzzle using the newly acquired portal gun.
 Behind the scenes
This level was intentionally designed to ensure that players realized that each portal could be used as both an entrance and an exit.
 Test Chamber 04
More buttons and boxes. At the end of the chamber, GLaDOS states that the next test chamber will not be monitored, as "part of a required test protocol."
 Test Chamber 05
This time, there are two boxes and two buttons. GLaDOS reveals that "our previous statement suggesting that we would not monitor this chamber was an outright fabrication." This begins to hint at her malicious behavior. She then states says that "We will stop enhancing the truth, in 3, 2 -" but is cut off before she can finish, effectively nullifying the statement.
It is possible, in both Portal and Portal 2, to get trapped in this chamber. This is accomplished by finishing the puzzle and shooting a portal into the next room, then stacking the cubes and camera to reach the orange portal, and throwing a cube into there. If the player then reshoots a blue portal, the player is now trapped, as there is a closed door with no way to open it (a cube is now behind the door), and no portals on the other side. This will cause GLaDOS to say "You're not a good person. You know that, right?" (a line she uses in the post-escape levels). If it's done in Portal 2, both cubes are "fizzled" as soon as one cube enters the portal and the Announcer says "You have trapped yourself. Congratulations. The exit door is now open."
 Test Chamber 06
Here, the Aperture Science High Energy Pellet is introduced. Also introduced are brown walls, which are non-portal surfaces, that are seen throughout many of the following test chambers.
 Test Chamber 07
More energy balls, also moving platforms known as "unstationary scaffolds" are introduced.
 Test Chamber 08
Acid pits are introduced, in combination with moving platforms and energy balls.
 Behind the scenes
This was to be the first test chamber to feature energy balls. This chamber was shown to introduce too many new concepts at once and players were often frustrated. So the two preceding simpler test chambers were inserted before this one to introduce the energy ball puzzles more gradually.
 Test Chamber 09
A so called "impossible" test chamber, with buttons and cubes used.
Later in the game, this chamber is revisited illicitly, and must be solved in a different way, as the cube is no longer available. As well as having access to both sets of portals (instead of blue-only originally) Another way to solve the test chamber with dual portal gun is placed over a piece of the dispenser through a portal
 Test Chamber 10
A new concept is introduced: flinging. As GLaDOS says: "Momentum, a function of mass and velocity, is conserved between portals. In layman's terms, speedy thing goes in, speedy thing comes out."
 Behind the scenes
The concept of flinging was one of the hardest concept for the developers to convey, so this test chamber went through more iterations than almost any of the other test chambers.
 Test Chamber 11
Chell acquires the full Aperture Science Handheld Portal Device that allows her to place orange portals as well.
It is possible to get out of the map by shooting a portal at the energy ball receiver and hopping in the connecting portal. The portal hits the wall and ignores the object, which causes the player to fall out of the map.
 Test Chamber 12
More flinging, now with more freedom allowed by the upgraded device.
 Behind the scenes
This chamber was designed to retrain the player in using the fling maneuver, now that players could manufacture two portals at once.
 Test Chamber 13
The first real challenge. Combines buttons, boxes, energy balls and moving platforms.
 Behind the scenes
The previous stages were constructed to introduce the basic concepts of Portal in small, easily-understood segments, and this stage collects them into one place as a sort of 'final review'.
 Test Chamber 14
Another challenge. Combines buttons, boxes, energy balls, moving platforms (moving vertically in this case) and acid pits.
This chamber can be completed without even leaving the first room, thus omitting most of the level. It is more likely a feature rather than a bug, as the Challenge series version of the chamber requires using only 2 portals to get the Gold, which is impossible while using the ‘fair’ way. To do this, you must first place a portal on the floor next to the Victory Lift, then ascend the stairs on your left. At the top, turn around and look down. When the stairs have fully retracted again, fire the other portal onto the surface the stairs form. Jump down and you will emerge through the portal by the lift. This may require several tries, as it is often hard to turn around and land on the lift itself, due to the fact that the first portal will most likely make you emerge facing away from it. Once you are on, simply walk through the grill and into the elevator.
 Test Chamber 15
A quick but also very difficult test. It involves the use of the Emancipation Grids that are used outside of just leaving test chambers and being used in tests. Often frustrates players because they are unaware of the grid's ability to close portals.
 Test Chamber 16
"Due to mandatory scheduled maintenance, the appropriate chamber for this testing sequence is currently unavailable. It has been replaced with a live-fire course designed for military androids. The enrichment center apologizes for the inconvenience and wishes you the best of luck."
This level introduces turrets, three foot tall gun-toting tripods that speak cute phrases when they see a target or are tipped over.
The player finds the first Ratman den, hinting at the reality of the tests, as well as the fact there there are other people still trapped in the building.
 Test Chamber 17
Notable because of the appearance of another character: the companion cube. At the end of the chamber, Chell is forced to "euthanize," or more bluntly, incinerate the Companion Cube.
By using noclip, players can find a portion of Test Chamber 17 used in the Advanced version of the map in which the player is required to fling through a glass hole along with a Companion Ball used in the advanced chamber.
 Behind the scenes
This chamber was actually originally a long level called the "cube run," where players would take the cube through a number of challenges. However, playtesters would leave the cube behind, so they put a heart on it and had the AI keep on talking about it. There is also another Rattmann room hidden in this chamber, filled with writing and photographs indicating that the Rattmann became obsessed with the cube in some way, and developed a seething hatred for GLaDOS after its destruction.
 Test Chamber 18
This is often considered the hardest chamber. Turrets reappear here. At the end, GLaDOS says that "the next test requires exposure to uninsulated electrical parts, that may be dangerous under certain conditions," which comes into importance later.
At the beginning of the chamber, there is an audio glitch: GLaDOS was supposed to say "You will be baked (bzzzt) cake," as indicated by the subtitles. However, the in-game quote is not censored: "You will be baked, and then there will be cake." This is most likely a development oversight. This quote also foreshadows GLaDOS' betrayal of Chell.
 Test Chamber 19
The last chamber. A simple energy ball trick is required, followed by a time-limited sequence on a moving platform.
As the platform rounds a corner, awaiting the player is not the long-awaited cake but a large incinerator. Chell escapes via portals, much to GLaDOS' apparent displeasure; she briefly babbles electronically. She stutters on her "I," revealing her true nature, before regaining control and attempting, unsuccessfully, to convince Chell that this was "the final test, where we pretended we were going to murder you." Chell escapes into the bowels of the Enrichment Center.
After the final test, Chell is "sent to receive her cake," but is plunged into a furnace (to "be baked," as GLaDOS had accidentally admitted numerous previous times, with glitches resembling Freudian slips). She manages to escape into a maintenance area and GLaDOS begins to show agitation, claiming first that it was the real final test, and then that it was just a joke. She tells Chell that there will be a party held in her honor and urges her to "assume the party escort submission position" on the floor. Chell flees into the facility's numerous maintenance areas, and GLaDOS becomes increasingly agitated. Roughly the second half of the game continues behind the walls of previous stages and the inner laboratories of Aperture Labs as Chell follows eerie notes written on walls and uses the ASHPD to work her way through the facility. Finally, Chell confronts GLaDOS, a large machine hanging in a cylindrical shaft. (Near the entrance to the room is a desk with a red phone. The developers stated that there was someone stationed at the red phone in case GLaDOS showed signs of independent thought; apparently, they weren't able to make the call, as the wires were cut.) As Chell destroys critical components of the machine, GLaDOS' personality degenerates. During this confrontation, GLaDOS makes several comments that something has gone wrong in the outside world, and that she is the only thing keeping the facility from "them," possibly the Combine, or something else entirely. After Chell destroys the last component, some form of portal opens above them, tearing apart GLaDOS and sucking in Chell. Chell wakes up at the Enrichment Center's gate, among the wreckage of GLaDOS, as debris rains down from the massive, unseen portal. Then the Party Escort Bot thanks Chell for assuming the party escort submission position, and brings her back inside the facility.
The final cutscene zooms up and through various areas of the Enrichment Center, ending in a darkened room showing the promised cake surrounded by various metallic glowing orange spheres similar to those that were installed in GLaDOS. Some of the eyes activate and a robotic arm puts the candle out. The credits roll as GLaDOS gives a concluding report about Chell in the form of the song "Still Alive." It reveals that GLaDOS enjoys the companionship of the test subjects and that it felt a particular fondness for Chell. The song seems to hint that GLaDOS is, indeed, still alive.
When the credits and song end, the player is greeted with a new background: a cake with a lit candle on a desk next to a radio on a table as well as the same red phone (which has its buttons shaped like the Aperture logo). If the background is left alone for long enough, it eventually pans to show the Companion Cube on the floor to the right side of the desk.
- This section of the game does not bear any official title. "Escape" is the map prefix, and the game files refer to "Behind the Scenes," or "BTS." Within the game itself it's listed as part of Test Chamber 19.
- The Nice Little Jellyfish is an Easter egg found in this level. It is located in a small office room overlooking Test Chamber 09 that is found soon after the fan blades in the maintenance areas of the facility. It is a faint pencil drawn image on the wall of a smiling jellyfish with the text "nice little jellyfish" under it.
- The Big Bad Jellyfish is an Easter egg found in this level. It is in a side room on the corridor just before you find the first Rocket Turret seen in the game. It is a pencil drawing on the wall of an angry looking jellyfish with the text "THE BIG BAD Jellyfish WANTED" under it. Both Jellyfish may have been drawn by Doug Rattmann but there has been no confirmation of this.
- GLaDOS' destruction mirrors the death of the Nihilanth, as they both floated up into an ominous white glow, whilst pulling the protagonist up into the air along with them.
- Test Chambers 00, 01, 02, 04, 05, 19 and the Relaxation Vault make an appearance in Portal 2, however Test Chamber 19 does not need to be completed again.
- In the meeting room behind the window adjacent to the "nice little jellyfish" Easter egg, there appears to be a PowerPoint presentation entitled, Competing with BLACK MESA.
Traps in a typical maintenance area of the Aperture Laboratories, mirroring traps seen in the Half-Life chapter Residue Processing.