Chapter 3: The Return
Test Chamber 09
After entering, step on the Aerial Faith Plate, and repeat for as long as GLaDOS does her thing. Once she lowers the ceiling above the plate, shoot a portal onto it, and one on the horizontal strip on the ceiling ahead of you. Use the Faith Plate to jump through it, then walk forward and to the left. Fire a portal onto the furthest angled platform - the one which is aimed at the elevated area marked with an arrow - keeping the portal you placed on the ceiling above the Faith Plate where it is.
Use the Faith Plate again to launch yourself through the angled platform and onto the raised area, where you'll find a button. Press it to release a Discouragement Redirection Cube on the area across from it, then repeat the jump on the second angled platform to reach it. Once you have the cube, use it to aim the laser at the free-standing wall across the way.
Using the third angled platform, which you'll find parallel to the wall raised area which houses the exit door, repeat the Aerial Faith Plate jump again. Once you've made it up to the exit, you still need to trigger the switch with the laser. Fire a portal on each side of the free-standing wall, so that the laser runs through it and hits the switch.
Test Chamber 10
After entering this chamber, walk straight forward without stepping on the Aerial Faith Plate. You will come to an elevated platform with a cube on it, cut off by a deep gap. Fire a portal at the bottom of the gap, and another at the very top of the wall behind you. Jump through your portal in the gap to launch yourself onto the platform and retrieve the cube. Drop down to the gap and back through your portal, bringing the cube back to the main floor.
Step on the Faith Plate to launch yourself to the next level. Use the cube to block the laser, which will lower the barrier you just flew into. Go back to the Faith Plate and launch yourself again. This time, you'll go a little further. On the second level, you'll find a button which releases a Discouragement Redirection Cube. Press it, and the cube will be launched over to you. Grab the cube, and drop down to the first level. Move the first cube out of the way, and interrupt the laser with the Redirection Cube to aim the beam at the opposite wall. Grab the first cube and go back to the Faith Plate to launch yourself back to the second level. Fire your other portal into the wall near the button which released the Redirection Cube, causing the laser to hit the switch across from it.
Now just grab the cube and return to the Faith Plate. This time you'll bounce all the way to the third level, where you can place the cube on the button and exit the chamber.
Test Chamber 11
Fire a portal at the surface which interrupts the light bridge, then one on the wall across the gap. Walk through your portal and press the button on the other side, which will cause a continuous stream of cubes to drop through a glass enclosure on the wall.
To get one of the cubes, return through your portal to the area where you entered the chamber, and, facing the gap, fire a portal onto the wall which juts into it on the left. Your first portal should still be connected to the bridge - walk through it, and to the end of the bridge, which should take you through a small passageway by the falling cubes. Now turn to your left, and fire another portal on the far wall, moving the bridge below you, and allowing you to continue walking into the passage. Finally, turn to your right, and fire another portal, reorienting the bridge again and allowing you to catch one of the falling cubes.
Finally, return through your portal with the cube, and fire a portal on the right wall, so that the bridge connects to the room containing the button, which you'll see blue dots leading to on the left. Walk through your portals and onto the bridge, jump down to the platform and place the cube on the button. To get back, reposition the bridge so that you can jump onto it, and walk back through your portal, then use the bridge to cross the gap again and exit the chamber.
Test Chamber 12
After Wheatley talks to you and the door opens, enter the chamber. The button on your right drops a cube into a pit. Place a portal above the light bridge, then onto a wall to reach it. Once on the bridge, shoot a portal at the end of it, then walk through it to return to the entrance.
Create a bridge across the gap, so that the cube falls onto it when you press the button. Grab the cube and walk across the bridge the other way, crossing the larger gap. You'll have to shoot another portal behind you to lower the bridge while you're still on it - be careful that the cube doesn't fall off when you do this. Once you and the cube make it across, place it on the button to finish the test.
Test Chamber 13
Dispose of the first turret by picking it up and dropping it on its side. To get behind the next turret, you'll have to shoot a portal on the wall behind it. The last set of turrets - two in the hall, and several in the room behind it - must be deactivated by dropping cubes from the ceiling.
Once they're all down, put a cube on the button to continue.
Test Chamber 14
Pick up the redirection cube and aim it directly through the first switch, so that the beam is perpendicular to the wall behind it. Shoot a portal at the point it meets the wall, then another on a side wall so that the beam crosses through all three switches.
Test Chamber 15
Run past the first turret and knock it over, leaving the light bridge blocking the row of them to the right. To get to the back of the room, use the light bridge to block the turrets ahead and run for it. Once you're at the back, fire a portal at the top center of the rear wall, directing the light bridge over the platform - it will catch you when you launch yourself from the Faith Plate.
Once on the elevated bridge, run back to the front of the room, and drop onto the cube surrounded by turrets. Once in the middle, you can easily knock the turrets over and grab the cube. Be careful - once you put the cube on the button, two more turrets will be revealed in front of the exit. Portal behind them to escape.
Test Chamber 16
Stand on the button to activate the laser. Fire a portal where it meets the floor, and another on the wall of the hallway containing the first batch of turrets. The laser will destroy them. Grab the regular cube, and use it to hold down the button, then use the Redirection Cube to destroy the rest of the turrets. Once they're dead, aim the laser at the switch and walk to the exit.
Test Chamber 17
The cube on the light bridge can be retrieved by firing a portal at its end, another on the wall, and walking through it. Once you have the cube, move the light bridge to the wall across from you, and walk through your portal onto it. Position the cube so that it blocks the laser, and sits over the button. When you block the laser, the elevator in the back of the room will lower. Step onto it, then move the light bridge, causing the cube to fall onto the button. The elevator will rise, and the door will open.