The quest is given to you after talking to General Silverman, given you were not sent to jail because you were wearing a vault 18 suit or were being rude to the general.
Once the dialogue is over, you will hear a vertibird flying over followed by an explosion. Silverman then tells the player to run to meet Jameson out in front of El Casino.
Once there you will notice that Survivalist raiders are attacking Union City. You need to end the battle by killing the raiders that are by the Union City gates. The next objective will appear once you have killed all three Raider commanders and their forces. You will need to head back to General Silverman.
Silverman will order you to activate the power at the nearby Euripides Power Relay. He doesn’t know why the plant suddenly went dark, but he needs the power lines restored. He assumes it’s an easy mission, but offers you NCR Dog Tags that allow you to hire NCR Mercenaries.
He will also tell you to pay a visit to old Ooga in the far Xiabula Hills. They also vanished from peace talks a few weeks ago, and no one’s heard from them since. They have a water line near their tribe that supplies Union City off the old reservoir from before the war. The player’s orders are to restore the flow of water to Union City.
If your science is good enough you can restore power from Union City and skip the Euripides quest altogether.
You can also convince Marius to make a power supply with a Recharger Rifle and a lenght of cable that can be bought altogether at Molly's Junk Shop
Otherwise you will have to go to the power station.
Once you reach the power relay, you will notice a battle between Savages and Shi raiders. You may join the Shi (led by Kieva Nanjima) in fighting off the Infected Savages, or attack both. It is possible to eliminate the enemies without fighting them, if you sneak into the old Jet crashed on a nearby hillside and program its targeting computer to fire an old missile barrage at the power plant (45 repair and 65 science required).
They player needs to hit a switch to activate the plants and then finish all waves of enemies before leaving. Alternatively you can ally with the raiders to defeat the savages if your speech is good enough.
In order to restore the water, you must head to Xiabula. On the way, you will encounter several Infected Savages, whose clothes and weapons transmit the virus that can knock out the player by draining their health until they “die”. If the player equips their weapons or armour, it drains 5 health every 30 seconds until you hit 10% health, you then pass out and the screen goes black. You are teleported to a dream sequence where they must fight off death once again. When they awaken, they will be in a tent in Xiabula, their companions gathered around, and Ooga’s Son tending them.
If you don't get knocked out by the infection, you can head towards the camp normally and talk to Ooga when you get there, if you are infected (but not yet passed out) you will be cured at the camp.
Once you reach Ooga, you can have a long conversation with him and will eventually learn that the water supply has been infected. You can either fix the supply with a high science skill by connecting another water line or carry pure water to union city manually (it will take several trips).
Once both objectives are complete, head back to Silverman. He’ll tell you about a secret objective to outrun the Raiders to Fort Daggerpoint, and their plan to secure all the weapons there. After this, the quest will end and Journey to the Deep will be activated.