This page contains walkthrough for the third main quest in Fallout 76. Thirst Things First starts automatically after you finish First Contact main quest. Your goal is to look for a responder, and to purify water samples.
Locate Kesha McDermott
Your first objective is to look for a responder called Kesha McDermott. Use the computer - the same you have used for volunteering during First Contact mission. Find Kesha McDermott on the list. Leave the building and head straight towards the river (the best way to do that is to follow the quest marker).
Find Kesha McDermott's corpse - the body is lying on the shore. Loot her body - take Water Testing Kit and other items.
How to examine the water sample?
Your next objective is to analyze two water samples - collect them from the river and from the water pumps. The locations are marked red in the picture above. Approach the river or the pumps and press the correct button - your character will collect a sample. The first sample should be taken from the river, the same where you have found the corpse. The second sample must be taken from the pump on the nearby field.
Take the samples to Flatwoods - go to the white church. Inside, you will find a Diagnostic Terminal - use the device and select the option to analyze the water samples.
Your last objective is to boil water. Go to a Cooking Station - the nearest station is marked in the picture above. Get two units of boiled water. You need dirty water (your samples) and a piece of wood - you should have this item in your inventory (i.e. after your visit to the sawmill).
Go to the computer inside the nearby building (the one where you volunteered), and check the volunteer status. This ends Thirst Things First - you receive caps, ammo, Stimpacks and crafting plans. You also unlock another main mission called Second Helpings.