Portal Stories: Mel

Portal Stories: Mel is a free single-player mod for Portal 2 which tells a story of a female Olympian named Mel who volunteered in 1952 to be a test subject in Aperture Science. The main story takes place between the events of Portal and Portal 2 when Mel tries to escape the Aperture Science laboratories together with a personality core Virgil.

The mod is available on Steam as a stand-alone game, although you still need to own Portal 2 to play it. It features original voicelines and assets, new gameplay mechanics (fire and water, Human Emancipation Grid) and 22 new levels.