Portal is a puzzle game where you manipulate the world around you to move through increasingly complex chambers. By creating two connected portals- if you walk into one, you walk out the other- you must move yourself around the rooms to escape. It stands out for how this mind-bending system allows you to travel around in unexpected ways and find ingenious solutions.

You are in a science laboratory where you seem to be watched and tested by the artificial intelligence GLaDOS. You have a gun that can create a pair of magic portals on any surface: one orange, the other blue. If you walk through the blue portal, you emerge from the orange one and vice versa. Through a series of increasingly ingenious (and malicious) test chambers, you must use your gun to escape. As you progress, the stages become ever more challenging to solve, requiring lateral thinking and experimentation to find a way out.

It’s a mind-bending challenge, but the real interest is the darkening tone of GLaDOS, who seems bent on foiling your progress, and the signs of previous test subjects spotted behind the scenes. As GLaDOS’s dysfunction turns to straight-up sociopathy, you must take on challenges with her tormenting discouragement ringing in your ears. The isolation is heightened by the introduction of a lifeless companion cube, which mocks your friendless predicament.

The result is a game that offers a mind-bending challenge paired with an enticing story only hinted at; behind the puzzles is the bigger mystery of where you are, and what these tests are for.