probably the best Puzzle game ever created along with tetris
you are chell a lady who wakes up in a facility used for training, you will be given a handheld portal gun that will allow you to create two portal, one is blue, and one is yellow, when you cross the blue portal you will go to the place that the yellow portal leads you, and opposite is also correct .
you will use physics to solve ypu puzzle like jumping or going through tight places or even making a turret falling down to their end, the objective of all these puzzles is to reach the exit door, you will encounter many other things that I won't spoil for you in order to play the game with all the atmosphere and mysteries and surprises .
one more things, if you finnish it google it a bit to discover many extra things that you might encounter.