SpawnPoint

In Portal, players control a test participant in the Aperture Science Enrichment Center. Using the "Aperture Science Handheld Portal Device", players perform a variety of tests, such as creating portals to knock over turrets and other objects or moving to a previously unreachable area. Guided by an electronic female voice that is at once both comical and subtly threatening, players must either complete these objectives or fail the test. Failure or refusal of a test likely results in death, although it may not be permanent. At the end of the video the computerized voice ominously says "As part of a previously-mentioned required test protocol, we can no longer lie to you. When the testing is over, you will be... missed"; in another, more recent video, the voice states that "you will be baked, and then there will be cake", perhaps alluding to the flame-filled areas seen in the original trailer. The Leipzig video, however, states that "an  associate will be dispatched to revive you with peptic salve and adrenaline" in case of consciousness loss, hinting that some care is taken to keep the test subject alive at least until the tests are over.

Portal has been confirmed to be set in the Half-Life universe, and it appears to take place in a "Combine portal user training facility", according to Shacknews. Certain aspects similar to the Half-Life universe can be noted in the official trailer, such as the turrets, which operate and sound similar to those that the Combine implement, though this may only be due to the use of sounds from Half-Life 2 as placeholders. Also, orbs like those that the Combine use in the Citadel are seen, with sockets similar to those seen in Half-Life 2: Episode One. Unlike the orbs in Half-Life 2, though, the orbs in Portal will instantly kill the player if they get too close to them or attempt to grab them with the ASHPD. The Portal Gun functions much like the gravity gun (from HL2), with the added ability of creating portals. Like the "Gravity Gun", it can pick up and hold inanimate objects, though its range appears to be limited.

According to Doug Lombardi, the protagonist in Portal is a yet-unintroduced character who will play a role in future games of Half-Life. Though Lombardi has hinted that this character might make an appearance in Half-Life 2: Episode Three, this is not a certainty. It was later suggested in an interview article from 1up.com with Portal level designer Kim Swift that the jump-suited male character from the preview video is a placeholder character; the walkthrough videos from the 2007 Game Developers Convention also describe the jump-suited male as "not the final character." Assuming that it is correct, it could be hypothesized that since the trailer is in the form of a training video, the jump-suited male is intended to represent a demonstration of the portal technology, recording a video that is being viewed by test subjects as they are indoctrinated into the program (It could also be that the jump-suited male is being used in the place of the female protagonist so as to keep her identity secret until the release of the game). It was confirmed in a GameInformer article and even through a screenshot from ComputerAndVideoGames.com that the protagonist would indeed be female.

Information about the Aperture Science Center will not be revealed in Portal, but will rather be explained in Half-Life 2: Episode Two. However, the background of the female protagonist and her reason for being at the Aperture Laboratories will be explained during the game.