You start this chapter watching a cinematic of you being about to be lobotomized. After the cinematic ends, you will be in front of a table full of pictures. Before looking more closely at them, pick up the newspaper on the table and use any keys you may have in your possession, as well as upgrading a the chair with any Green Gel you may have. After this, go back to the locker room and look closer at the photos on the table. A noise like breaking glass will fill the room as the ghostly hand appears on the last picture. Sebastian will comment on this, and the wheelchair will roll towards you. You will also notice that the window and the mirror by the sink no longer have any cracks in them. Head to the mirror at the very end of the other hallway and you'll see that that mirror also cannot be used. Save at the desk if you like, and then follow the new corridor to the right of the ornate mirror.
Whatever you see is just a hallucination, so it can't hurt you. At the very end, you'll see Leslie crouched on the ground, and when he turns his head in your direction, his face is replaced by a mirror, which will automatically eject you out of the save room, and into the basement of a ruined house.
Climb up the boxes to reach the next room, and pick up the supplies there. Exit this house and follow the path to a house on your right. Notice that there are some hanging boxes. Shoot the boxes, and they may drop loot. The same can be done in the next house, and behind the hanging body is a morgue key.
Now that the houses have been cleared, head to the gate to trigger a cutscene where you find your partner, Joseph. After the scene, you have to defend him while he opens the door. The Haunted will stream though the windows, and you have to keep them off him for about 2 minutes, because if he is interrupted (by an attack), the sequence of his opening the door will take longer. Note that there are two explosive barrels in the room. Try to use these as efficiently as possible, as you only have the two barrels, so try to gather the zombies around them before you shoot the barrels. Once he has the door open, quickly grab whatever items you see before leaving the room, as zombies still filter in through the windows. Once you pass through the door, it gets locked off by flaming debris. For now, you are safe from enemies, so heal with any Syringes you may have and grab the small amount of supplies before you follow your partner again. When he has to open another locked door, more enemies will show up, but they will be dropping down from the floor above. Again, you have two barrels, so use them as you did when you were upstairs. Keep an eye out for fat zombies or the zombie priests, as they deal out more damage and are harder to kill. When he finally has the door open, rush with him outside and a short cinematic will play of you locking the zombie horde in the burning house.
Now, you're in a massive ruined warehouse. Open the silver gun case to get your hands on a new weapon: the Sniper Rifle. Also on the table is a syringe, and to the left of the table is a mirror. Make sure you use it and upgrade via gel and key(s), as once you move past the initial area and try to come back, a sheet of tin will fall from above, smashing the mirror, so be sure to save, as well.
You have two options for moving forward: the left path and the right path. The left path has a patrolling zombie with a crossbow equipped with exploding bolts, and alerting him will summon the two to three zombies beyond the door he guards. The right path has two zombies on the ground, though some will come down from above to replace them if they are killed silently.
Note that if Joseph gets injured, you can heal him (via on-screen instructions), but, like you, he can die from too much damage, and if he does, the game reloads the autosave. He will generally stay close to you, but may also stay back a ways. He is also without his pistol, but uses a two-handed axe, instead. The axe has a fairly high chance of instantly exploding the heads of enemies, much higher than whatever chance you have with your weapons.
Take the right path, as it is easier to kill the enemies silently. If one happens to spot you, Joseph will generally take them out, but if he doesn't, a melee attack or pistol shot to the head when they are near a hole in the ground or at a height will stagger them enough to kill them with a fall and little fuss. Initially, there are two on the ground and three up top, so try to kill the one in the little central building with a sneak attack (or the strategy above), then sneak to avoid triggering the bomb and sneak kill the one patrolling near two levers, which you can't use right now. With those two gone, raid the boxes and disarm the bomb, as well as picking up the green gel and ammo. Head up the ladder by the broken yellow door and notice the two zombies on the far ledge. There are also two more zombies on the far left, but they will not notice if you attack their buddies over here. In order to preserve ammo, get the closest zombies' attention, then run back and let your partner kill them.
At this point, you'll notice lots of harpoons in the ground. Move close enough to them and Joseph will warn you about snipers. Far across the way, on a tower, are four hanging boxes which each carries a zombie who will open his hatch at regular intervals, (provided you are close enough), and operate a sort of automatic crossbow. The clumps of bolts in the ground (while they can't be picked up) will show you where the general crosshairs of a particular sniper box is located. DO NOT snipe at one of the boxes yet, shooting one will summon more enemies for you to handle. Instead, avoid the range of the snipers and take out the two zombies on the walkway headed to the right side.
Note that your partner may panic when being shot at by the snipers, and will fall off the walkway, hanging on the edge. Help him up (via activating him) to earn an achievement, and take out the lone zombie in the house closest to you. Be wary that he throws Molotov cocktails, and if you're hit, you'll take fire damage over time. Kill him however you please, then sneak in, because there is a bomb right inside the door, to the right. Disarm it and quickly sneak out the way you came, because a zombie comes up some stairs to briefly patrol the floor, before turning and walking down the stairs again. If done properly, you can sneak up behind him as he walks down the stairs and get in a sneak kill. Right by the stairs down is another clump of bolts, warning you that this is where a crosshairs is aimed. Wait for the sniper to shoot, then sneak down the stairs when he closes his hatch.
When down the stairs, there should be two guys here; one by the pile of lumber, and another by the generator. Watch out for the bomb on the wall. You can attempt a sneak kill, but one will likely notice you while you kill the other, so shoot one to get their attention, then have them run after you up the stairs and blast them both in the face with the Shotgun. When they're dead, go back down and disarm the bomb (you'll need the parts for later in the chapter), pick up the ammo on the shelves, then head down more stairs to find another bomb. Disarm this one as well, then ready a flash bolt and burst out the door, as the guy with the crossbow is still patrolling the door. Hit him with the bolt, then go behind for a sneak kill. Head back inside and flip the lever on the generator, and the game will show you the two levers you passed earlier flip up.
Head over to the levers, and now you have a choice, which is irreversible (you can't flip both levers): the right lever opens up a room with lots of Green Gel (including potentially the Gel left behind by the enemies within), but the room also happens to have three of those zombie priests who stream out and try to kill you. The left lever opens up a room without enemies, but has two treasure chests, plus other supplies. Note that none of the chests are rigged with a bomb. Make your choice, then scoop up the goods and head up the ladder to reach the top of the central building. On a little ledge (you have to hop off onto it) below the bridge is a large bottle of Gel. Grab it and then climb up onto the bridge again and stand where there are harpoons. This will make the sniper shoot, so quickly take him out with the sniper rifle, then run along the planks so you are above the broken yellow doors. Joseph will say something about enemies, and the game will prompt you to look closer at the enemies streaming to your location. It is highly important to stay where you are, as the zombies have to climb ladders and run quite far to reach you, acting as a "chokepoint" of sorts. While it seems that the waves (which consist of about two to three) are endless, the game will run out of enemies to spawn eventually, and since your partner can take care of them in one or two hits (rarely three), this is a great way to "farm" items, since enemies have a small chance to drop items anyway. Some of them carry Torches, which can be picked up to hit and subsequently burn enemies, thereby saving Matches. If you're playing Casual or Survival, the eye at the top center of your screen will show you if you've done away with all the enemies yet. If its gone, then there are no more enemies left that will filter over to your and your partner's position. Take this fine opportunity to pick up whatever things the zombies left behind; could be Gel, ammo, even sometimes a syringe or two.
Walk to the other areas with clumps of bolts in the ground and wait for the sniper to open his door, so you can hit him. You'll know when you've killed him, as every sniper that has been killed makes the box that they are in fall off the tower. Ideally, you will want the last sniper you face to be the far right box, because the game has a boss waiting to come out of the tower. Kill the last one, and the game will save a checkpoint, and walk down the stairs to face the double yellow doors. All other doors that lead to different parts of the complex shut at this point, so make sure you've got everything you want, because there is no going back once the boss is summoned. Your partner also gets sealed off from you, so you'll have to battle the boss on your own.
Approaching the explosive barrel will make the doors ahead of you open, and the boss is none other than another Sadist. Unlike the one you faced in Chapter 3, this guy performs insta-kills on you far more than his counterpart, so stay well away. To get rid of him, keep on the move and utilize the crossbow to full effect. Lock him in place with freezing or shock bolts, then let him have shotgun blasts and harpoons to the head, and rinse and repeat until he bites the dust. Harvest the Gel he left behind, and grab whatever ammo was lying around before heading inside the foot of the tower.
Walk up the stairs with your partner and he will comment about why the elevator is not working: there is a body stuck between the doors. Burn it away with a match, and before getting in, go around the corner for a map fragment, and enter the door for a personal document. Enter the mirror and grab the newspaper off its usual place, then upgrade with Gel and save before exiting, as its a long way before you meet another mirror. Head back to the elevator and get in, riding it to the top.
After a cutscene, use the sniper rifle to shoot the guys bringing your partner to be executed. Walk around the corner of the bridge and he'll drop a plank so you can get up on the other side. After another cinematic, you'll have to protect Joseph for a third time while he gets the crank to work. However, there are not nearly as many enemies this time, and at first they'll all be bunched up on the other side, which makes them perfect for a Grenade toss. If you missed the opportunity, get them as they come for you one or two at a time, but watch out for the guy who likes to stay across the bridge and throw axes at you. Thankfully, his aim and reach of the axes isn't top notch, so he can be taken out with a few Handgun or a sniper rifle shot to the head. Once your partner has the gate open, run through for a short scene, and you'll find yourself in a marketplace.
The marketplace is free of enemies, so take as much time as you want exploring. There are boxes and barrels here galore to smash, and some ammo and Green Gel lying around. There is also a treasure chest behind the angel statue in the center of the marketplace, but it is rigged with a bomb. Open it halfway a few times to be able to disarm it, then open it fully and grab whatever's inside. There is also an audio tape and a key here. The tape is on a merchant stand, so follow the main path and look to the right. The key is found by going through a hole in the wall on the right and smashing the boxes.
When you enter the next area, a cemetery, your partner will get sick and a scene will play. When you gain control, explore the house. There is a map fragment here, some supplies, and a mirror. Enter it and grab the new newspaper off its stand, the missing persons poster from the tack board, and use whatever Gel you have and save. It is important that you save here, because a boss fight is coming soon.
Head back outside and the house will collapse. As your partner tries to spot for you atop a crypt, the ladder breaks and now you're on your own again. However, he gains a sniper rifle, which he will use periodically to help. There is one sniper across the way that starts to shoot at you, but Joseph should be able to take him out. Burn the "dead" bodies in the water and you'll see that two weren't dead after all! Follow the path and you'll find a set of doors that are opened via pressure plate. Stand on it to open the doors for ten seconds, then run in. Note that there is no turning back, now. Smash the boxes in the new area, and pick up the ammo at the corner. When you get all the way to the end, the game will show you a zombie opening a gate that concealed a giant. After you kill him, his twin will appear. These guys, while gigantic, are only as smart as your average haunted, but they hit a lot harder. There is a glitch you can exploit for the first one, where you can stand in the archway of the door where you entered and shoot him, and he can't get at you. Use the same strategy here as you did on the chainsaw brute earlier and when he's nearly dead, your partner will yell something about it. Shooting them in the head kills them quicker, and saves ammo, to boot. When the first twin is down, don't cheer, because his brother comes in hopping mad. Use the same tactics on him as his brother, but don't hide in the doorway, because he doesn't get stuck there. When both a gone, scoop up the mountains of Gel they left behind and head out the newly opened doorway.
Before you join Joseph at the horses, walk around back to find some Gel and ammo, then join him. Grab the document before staring at the hole. Look at the hole, then you will find yourself in the save room again. There are no new collectibles here, so use any keys and upgrade, then save and exit.
When you leave, you're in a catacomb with your partner. Walk down the halls and you'll find yourself in a room with tables and bodies. The tables are numbered. If you wait at the body conveyor belt, a sack will come by with a key statue. Talk to your partner when he examines the wall with the symbols. In order to avoid being shiskebabed when you leave, lower the bodies onto all tables but table 7. When you enter the hall, the camera will zoom out as you walk down, and when you reach the area where the spikes were, scary music will play, even if you got the combination right. When Joseph says, "I guess you got the combination right," the game will allow you to walk on.
The next room is a lab. Grab the document on the table and look closer at the diagrams there, showing the twins you just killed. There is also some ammo here. Leave, and in the next small room is a bottle of Gel and the last document for the level. The beast in the cage is asleep, so sneak around to the left side for a bottle of Gel. Leave the room and find a locked door. Only your partner can open it. When he does, the beast in the cage busts out and chases you, knocking your partner out and knocking off his glasses. Unlike the twins, this deformed dog is FAST. He also likes to hide in the bushes and then come out when you least expect it. As dodging him is hard (though it can be done), shoot a frost or shock bolt into the ground ahead of you so when he comes out, he will be stunned and you can attack without worrying. After it passes out, there is a short cinematic, then you have to get the glasses, which happen to be smack in the middle of the park. While you can run in and get them, killing the beast once and for all will grant alot of Gel. Your partner stays on the other side of the fence and will throw bottles every so often to distract the beast. If you're going to kill him, use the same tactics as you did in the first round. If you only want to grab the glasses (they glow bright blue), stun the beast with a shock or frost bolt, grab the glasses, and run back to the hole in the gate and exit. The choice you make is permanent; you can't change your mind after you go back through the hole. With the glasses in hand, a cinematic plays, and then walk up the church steps. Before going all the way up, head to the left and to an alcove with a large bottle of Gel. Now that it had been collected, walk up the steps to end the chapter.