Story line Dead Island 2 tells the story of a random survivor who discovers that he has immunity to a zombie virus that has hit Los Angeles. While trying to get out of the city, players meet interesting characters and during the passage of missions they find all sorts of secrets related to zombies and their immunity.

Quest for the Truth is a late-game mission in which you have to make your way through Ocean Avenue, fighting through hordes of zombies, and fighting a boss. However, this quest can be difficult as it is a higher level quest and players need to be prepared to complete it. So, here is the complete walkthrough of the mission "Search for Truth" in Dead Island 2.
This walkthrough contains a lot of spoilers
Content
Hunt Tisha

After completing the previous mission at the Santa Monica Pier and obtaining a blood drive for Dr. Reed, Jacob will return to the Serling Hotel on Ocean Avenue to discuss next steps. After a brief conversation, Dr. Reed will send you in search of his missing daughter, Tisha, after she left the safe zone without his permission.
Denise, the guard at the entrance, gives a hint that the last time Tisha talked about visiting the "Cup of Beans", which was once a cozy cafe. Here it is worth using a workbench to repair broken weapons or talk to a merchant to replenish ammo and supplies before returning to the streets.
From the hotel's main entrance, walk straight past the broken flower monument to Giallo and Long Pork Grill across the street. A little further is the coffee shop you need.

On the side with an open terrace is a place where players will find Tisha. Several unlocked doors lead to the Thalia building. Tisha will be in the apartment on the second floor labeled 2C, just behind a dead zombie with arrows sticking out of it.

Enter the apartment and enter the back left room to find Tisha. In a conversation with her, you will learn that she also does not trust Dr. Reid and considers his relationship with Konrad suspicious. So, Tisha sends you to the OSK store to contact Conrad using the OS system.
Contact Conrad at the OSK Store

The second goal of the "Search for Truth" mission in Dead Island 2 - go to OSK to talk to Konrad about her intentions. The OSK store is down the street from the Thalia building. Exit the way you came in and head down the alleyway in front of the Bean Cups cafe and apartments. The OSK store is farther away, in a building with floor-to-ceiling windows on all walls.
After entering the OSK store, go to the sKOpe station located on the back wall. Here you need to tell the server the secret code received from Tisha. There will be some zombies near the store, so you should take care of them in advance to avoid an unexpected attack. Unfortunately, the server is down, so you must find and restart it before using sKOpe.
Restart the sKOpe server

To restart the sKOpe server in Dead Island 2, you need to enter the staff area to access the server room inside the OSK store next to the front desk. The server room is locked, so you should look at the front desk for any information about the key card.
The note on the desk says that an employee named Henry will have the keycard. But, of course, Henry is now a zombie. You'll find him in the alley behind the back door of the OSK store. Kill Henry and take the key from his corpse. Prepare for battle as there will be other variants of zombies.

Return to the OSK store and enter the server room with the card key. Here you can grab the new Skill Card and the OSK Customer Service Scenario, then head to the far end of the room to pull the lever and turn on the electricity for the server. To fully turn it on, operate the switch on the server system opposite the power generator.

Now it will be possible to give the code words to the sKOpe system in the general store, but when you return, a horde of zombies will be waiting for you, so you will have to destroy all the enemies. There will be at least one screamer, which should be a top priority, because every time she screams, she attracts more zombies to her.

After defeating all the zombies, it will finally be possible to call the code phrase in SKOpe for the next steps in the Quest for Truth.
Head to Dr. Reid's Lab

The sKOpe recording reveals that another immune survivor, Noah, was working with Dr. Reed. Noah has been sent to Dr. Reid's laboratory on Ocean Avenue, and now you should go there too. A photo from a security camera will help locate the laboratory.
Exit the OSK store to the left and go down the stairs to the right. There will be an ambulance and a police car straight ahead. Turn left, go past the containers with explosives and go along the buildings. Then head towards the black truck and the yellow trash can where the road starts to go downhill. You'll know you've come to the right place when you see the white van and Tisha will contact you.

She will open the door to Bay 7, from which a group of walkers in a gray suit and a screamer will come out. It is necessary to kill them all in order to be able to explore the laboratory. After dealing with the zombies, enter through the service entrance and look for the entrance to the laboratory.
Behind the service entrance is a door locked with a triple lock. Here you need to break all the electrical boxes that connect to the lock. The first one is near the entrance to the garage on the wall next to the office. The second is located inside the office on the wall next to the door. You will find the last one next to a small balcony behind two fragile boxes that can be broken.

After all three power sources are broken, you will be able to go through the door and continue to Dr. Reed's lab for the final objectives of the Quest for Truth mission in Dead Island 2.
Defeat Noah's Exemplar

Just outside the maglocked door, there is a hallway leading to gray double doors, behind which is Dr. Reed's lab. Inside, you'll see a large glass cube with a supposedly dead Noah tied to a chair in the middle of the room.
Examine the lab equipment, including the sample container, transcoder, and the sKOpe and hemodrive on the table. Then use the hemodrive to boot the system and trigger a cutscene in which Noah comes to life and transforms into a larger and more terrifying Apex zombie. Players must defeat him and the other SZ-suited zombies that appear in the room.

Sample: Noah has two main attacks to watch out for. In melee, it has a slam and jump similar to a crusher attack that knocks the player back. Additionally, when the player keeps a distance, they will use a ranged attack, firing piercing spikes.
If you don't have a good melee weapon, the best strategy to deal with Exemplar: Noah is to use powerful ranged weapon. Use the explosives in the room to deal fire damage to him and also shoot at critical points. Whenever he performs a jumping shockwave attack, his claw can get stuck in the ground, allowing him to run up and land a few quick strikes.

When Exemplar: Noah is almost out of health, rage mode is automatically triggered - use it to complete the task. At this point, Tisha will enter the room and the player character will attack her in a rage similar to a zombie. Finally, Tisha knocks him out, and when he wakes up, Konrad and her group will be standing over him.

Conrad then reveals that there really is no cure and they are all part of an elite group of people called the numens. This unlocks Numen Skills, which work like Slayer Skills, but are significantly more powerful.

When the cutscene ends, the protagonist will return to the Serling Hotel to confront Dr. Reed, only to learn that he and Tisha have left the group and are heading to another lab. He then plans to reunite with Emma Giaunt and Sam B to go after them, after which the next chapter begins. Dead Island 2.