When you gain control, you'll play as Detective Sebastian Castellanos, flanked by a team that consists of a uniform by the name of Connelly, as well as two more detectives, Joseph and Kidman. Players might recognize the latter as Jennifer Carpenter, most famous for her role as Debra Morgan on Dexter.
With introductions taken care of, make your way into the Beacon Mental Hospital via the large front doors, then follow Joseph through the lobby and to a room on the left. This will trigger a short cut scene, and when it's over, you'll want to turn around and approach the monitor at the other end of the room. This will cause yet another cut scene to roll, an event you’ll want to get used to.
When you regain some control, follow the on-screen prompts to grab the knife from the nearby hanging body and cut yourself down. This will allow you to move around, and your target is the Ordinary Keys that are hanging near the table where the dude is cutting up a dead body. Wait for him to walk away, then sneak up and snag the keys.
Tip: Since you can't kill the big jerk, just grab the keys and escape.
Quickly exit the room out the way you came, turning right and using the keys on the nearby door. Head up the stairs and move along the catwalk until you trigger an alarm. When your adversary with the chainsaw starts chasing you, move in the only direction that you can and follow the on-screen prompt to make a daring escape into the grate.
After all the excitement has died down, you'll want to climb out of the pit of blood and body parts using the nearby ladder, then make your way around the outside of the room and through the door. Follow the corridor to a railing, taking a right and jumping into the water. You don't have far to go. Just climb back out using the nearby ladder and follow the walkway to the dead guy in the wheelchair. This is where you'll find the Old Note from Sewer - a handy document that gives you some hints on how to escape.
Continue to move along the walkway until you see an open doorway at the end and on your right. Go through it and up the ladder, then into the makeshift jail cell nearby. It's here where you'll find the Boiler Room Note, giving a little back story as to what went down at the Beacon Mental Hospital.
Head back out the way you came and look to the right. Interact with the panel next to the door to open it, passing through and following the hall all the way to the end. You only have one path you can take, so keep moving along until the scary dude with the chainsaw shows up, then follow the on-screen prompts to hide in the locker. Check it out, it's Alien: Isolation all over again.
When the big guy moves away, you'll want to step out of the locker, but from here on out you need to put your stealth shoes on and do some sneaking. Exit the room and follow the hallway, noting the room your adversary is occupying to your left. It's the door at the far end that you're heading for, so wait for him to turn his back and then start working your way there.
Tip: You can pick up and toss bottles to distract your enemy. This can buy you the time needed to sneak past him and make your escape.
Once the chainsaw wielding psycho is behind you, continue to make your way through the asylum and into a long corridor filled with wheelchairs and gurneys. This will trigger a scripted chase that requires you to make a run for the elevator and trigger the game's opening credits.
When the elevator ride is over, make your way through the corridor and out into the lobby where you began the chapter. Exit back out the front doors to trigger an extended cut scene and bring the first chapter to a close.