Portal: Prelude

The plot follows test subject Abby as she undergoes a series of experiments at the Aperture Science Enrichment Center. Throughout the trials, you learn about the facility's inner workings, the scientists' motives, and the reasons behind creating GLaDOS, an AI designed to replace humans and take control of the lab. Ultimately, the activation of GLaDOS becomes the pivotal event leading to catastrophic consequences.

The mod features extensive dialogue that fleshes out the personalities of Aperture staff and the atmosphere of the facility while it was still under human management. The story gradually builds toward the launch of GLaDOS, which serves as the point of no return for the entire facility.

Gameplay is rooted in the original game's mechanics but is significantly more difficult. You will navigate 19 test chambers along with "behind-the-scenes" sections and challenges. The puzzles often demand not just logic, but high reaction speeds, precise movement, and creative use of the portal gun.

The mod introduces new gameplay elements, including various energy and gravity fields, as well as reworked object interaction mechanics. Levels are more complex and less linear than in the original game, with difficulty ramping up toward the final trials.

Portal: Prelude stands out for its high difficulty, expanded lore, and wealth of content, making it one of the most famous and comprehensive fan projects in the Portal universe.