The Stanley Parable began its life as a wildly successful mod for Half-Life 2. It grew in popularity so much that a full stand-alone title was the ultimate goal.

Today, the demo for the stand-alone version of The Stanley Parable has been released. Next week on October 17, the full game will be released.
The Stanley Parable is a first person exploration game. You will play as Stanley, and you will not play as Stanley. You will follow a story, you will not follow a story. You will have a choice, you will have no choice. The game will end, the game will never end. Contradiction follows contradiction, the rules of how games should work are broken, then broken again. This world was not made for you to understand.

But as you explore, slowly, meaning begins to arise, the paradoxes might start to make sense, perhaps you are powerful after all. The game is not here to fight you; it is inviting you to dance.

Based on the award-winning 2011 Source mod of the same name, The Stanley Parable returns with new content, new ideas, a fresh coat of visual paint, and the stunning voicework of Kevan Brighting. For a more complete and in-depth understanding of what The Stanley Parable is, please try out the free demo.

The demo can be downloaded through Steam. It, from what I hear, does not contain story from the main game so you should be free to play it without fear of any spoilers for the full game!