Aperture Science is a secret weapons research organization in competition with Black Mesa. Chell is a test subject at the "Enrichment Center" facility who awakens to find an A.I. construct called GlaDOS requiring some tasks to be completed. They are testing the Aperture Handheld Portal Device (aka Portal Gun) which is able to create portals in space, allowing Chell to move from one point to another without actually crossing the distance. Chell is promised cake should she be able to overcome the obstacles and perform the necessary tests.
She has to use her new gun to fling, jump and fall her way through many puzzles. These include obstacles but also androids that fire at her. Although the game is played as a first-person shooter, a large amount of strategy and puzzle-solving is involved.