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, pushing buttons, or moving to a previously unreachable area. Guided by an electronic female voice known as "GLaDOS" 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." Still, the gameplay video from Leipzig Games Convention mentions that "an intubation associate will be sent to revive you [the test subject/player] with peptic salve and adrenaline" in the case of consciousness loss, hinting that some care is taken to keep the test subject alive at least until the tests are over. While at the same time there are hints to show that you are not the only subject in this laboratory.
Portal has been confirmed to be set in the Half-Life universe. Certain aspects similar to the Half-Life universe can be noted in the official trailer, such as the turrets, which operate and sound exactly like 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, again, potentially placeholders or possibly intentional (It should be noted that, unlike the orbs in Half-Life 2, 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 a Zero Point Energy Field Manipulator, with the added ability of creating portals - like the "Gravity Gun", it can pull and hold inanimate objects.
According to Doug Lombardi, the protagonist in Portal is a yet-unintroduced character (which many have believed to be named "Chell" due to embedded GCF file names) who will play a role in future games of Half-Life. Though Lombardi has hinted that this character might make an appearance in Episode Three, this is not a certainty. It was later asserted in an interview article from 1up.com with Portal level designer Kim Swift that the protagonist would be female, suggesting the jump-suited male character from the preview video is a placeholder character. This information, however, is asserted without a direct quotation by the article's author and appears in no other media. 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.
A dark undertone is revealed in the ApertureScience.com site (see below) in that the application questionnaire (accessed when the user types APPLY at the B prompt after logging in with the password PORTAL) asks such questions as how much the applicant can withstand pain before passing out, how they'd react to torture, if they felt anyone was watching them, how long they could stay awake, etc. The questionnaire ends chillingly with the user being told to wait at their workstation to be investigated by a superior. During the questionnaire, in some questions, the user can see letters dimming and (on one page) even see an ASCII art of a cake flash for a brief moment. By taking all the dimming letters in the questionnaire and arranging them in the order of appearance, it spells out THECAKEISALIE. Typing that at the end of the questionnaire reveals a hidden message from a distraught employee and a security camera video, hinting that Aperture is not all it seems to be, with the final line, "I don't think going home is a part of our job description anymore", before telling the user to press return if a supervisor passes by (which brings up a fake spreadsheet software image. The spreadsheet listing a curious item, INTUB-XLG, of which one was ordered, costing $974,999.99. The only other things on the spreadsheet are 50 units of flour and 75 units of thumbtacks.) There is also an option to try to INTERROGATE an employee, but are rebuffed as not having clearance with an error message stating: "Illegal attempt to initiate disciplinary action".