It is my understanding that the most sought after item from Hydroplant would be the Nova Trooper Data Module. That said, enjoy!
1) Begin by getting 5,000 faction with Taanab Hydro Station Corp (this will keep all of the Hydroplant NPC's from attacking you) - farm it at the Taanab Mining Facility POI. From there, head to the Hydroplant at /way -3598 -4327.
2) Turn right once you enter (back side of the structure). You'll come into an open room with a circular platform in the center. Go past this to the first Hydro Station Systems Admin NPC. Kill him, check system messages for the first code.
3) Go back the way you came, towards the entrance, but go past it to the far end (from start, turn left). Turn right at the end, and enter the code into the terminal on the far wall.
4) Exiting the first terminal room, go straight ahead until you come to a stairway down. Go down ONE floor.
5) Upon entering the door on the second floor, go straight ahead and turn right at the end. Go straight ahead again, to the second room, and kill the second Hydro Station Systems Admin.
6) Check your system messages for the second code, then proceed back to the door in which you entered the second floor. Don't go through that door, instead turn left into yet another room with a circular platform inside. Follow it around and through the next door, then turn right and enter the second code into the terminal.
7) Backtrack to the stairway again and proceed down to the third floor.
8) Follow the only hall straight ahead, then take the door on the right in that room, then follow all the way through to the room at the end and kill the third Hydro Station Systems Admin.
9) Check your system messages for the third code, and take the door opposite of where the Systems Admin was standing. Follow to the far end, and enter the code into the third terminal.
10) Backtrack to the room in which you made the first right, and kill the Hydro Station Director (boss) at the far end.
11) Congratulations, you've completed the Hydroplant!
If you've followed these steps exactly, then you should have 3,100 faction left with Taanab Hydro Station Corp. If you kill anything else inside the Hydroplant, you will automatically be attacked by any other NPC that is still alive. Overall, it should take about 45 minutes to an hour to complete if you're running it solo.
