Story

Throughout the Portal series, you take on the role of Chell, a young woman trapped within the Aperture Science complex. Chell is forced to solve tests of varying difficulty in chambers designed by an AI-gone-rouge named GLaDOS. During the early stages of the games, Chell is granted access to the Handheld Portal Device (commonly referred to as the Portal Gun), a piece of equipment developed by Aperture Science that allows Chell to create portals that are large enough for her to travel through. These portals act as gates between each other, allowing Chell to quickly traverse areas or reach normally unreachable places.

Portal
Portal has two distinct parts, the test chambers and the escape.

Act 1: The Test Chambers
The game begins with Chell awakening in a Relaxation Vault, where she is briefed by the voice of GLaDOS coming from speakers, before being released through a portal. The player is introduced to the testing process and mechanics, shortly thereafter obtaining the Handheld Portal Gun. As Chell makes her way through test chambers it becomes clear Motivating Chell with the promise of Cake at the end of the testing, Chell is put through tests of increasing complexity.

Portal: Still Alive
Portal: Still Alive is a Xbox 360 Live Arcade port of Portal, containing additional content in the form of new advanced chambers, inspired by Portal: The Flash version, with no additional story material.

Portal 2
Portal 2 was released simultaneously on PC & Mac, Xbox 360 and PS3.

Trivia

 * In order to flesh out the story in Portal 2, Valve originally intended to include an exhibition within the Aperture Science facility, one composed of different dioramas showcasing various aspects of the company and its philosophy. The dioramas were ultimately cut before the final release.