At the start of the game you have to find Eric Crown, a private investigator, and speak with him. You must use Interact With on Desk at the top of his office to get 5+ score(actually you don't have to do that, and you can play out the game without reading that riddle). This is the order of buttons you have to pick in Room II, on the box to open it:
Level 1: Cross,Apple,Skull,Cross;
Level 2: Apple,Apple,Apple,Apple;
Level 3: Skull, Cross, Cross, Cross;
Level 4: Cross, Cross, Cross, Apple.
After you open the box you will get a rifle. When you leave the room use Look At on the painting on the wall 4 times I think, until the character notices something odd with the picture. Then you use Interact with on the painting to open a secret passage. When you end in a room with a zombie, all you have to do is use rifle on him so he will die(you will have to do this later in the Zombie land, but on more zombies). After that you return to the pub, but since your assistant, Derek, won't talk to you in front of Ann, you have to get outside the pub, use interact on the car 4 or 5 times, so the alarm of the car activates. Derek will come and the dialog will trigger. Later on when you get into the labyrinth you have to go through the following paths:
level 1: Gorgon
level 2: Oil
level 3: Demon
level 4: God (because in one book in the library it says that you have to follow the path of the "god" G(orgon)O(il)D(emon)...well you get it now)
When you enter the laboratory you must read all the logs in the panel, and obtain the map.
You will find four new locations. Once you enter the Zombie land, it's all about being quick(you have to shoot all the zombies with the rifle before they get to you). In the supply base the code you enter to obtain "plastic pistol" is "always here". In the garbage land the only item you can pick up is torch. The torch can't work because it has no energy, so what you have to do is go to the lab again and use that torch onto the Lab power source. That's how the torch can work again. Now you have to use the torch onto the graves in the Tomb area. Once your character reads the name of the graveyard, and that the tomb you were in was the only tomb in that graveyard you can go to the lab again and use "communicator" in the panel to call your assistant so you could meet him inside the tomb. Later on after that short cutscene, when the dialog options appear you have to choose "Hey! There's a zombie behind you." so professor D would turn his back to you. When he does that use the plastic pistol from your inventory and shoot him.