Stop the Enclave

Stop the Enclave is an unmarked main quest in Fallout 2. To complete it, the Oil Rig's reactor must be overloaded, and the final boss defeated on the way out.

Quick walkthrough
Reach the oil rig, cause a reactor meltdown, defeat Frank Horrigan, and get out alive.

Detailed walkthrough
After reaching the oil rig via the PMV Valdez tanker, your first step will be to infiltrate the Enclave base.

Into the oil rig
Walk into the oil rig's entry hall, then walk down the stairs to the south. At this point, you must be wearing advanced power armor or the Enclave soldiers will attack you on sight. Even with advanced power armor, the presence of any companions will cause the soldiers and turrets to attack, so they should be left behind in another location to avoid alerting the guards. After exploring a bit, you will see people in cells. Talk to them and you will find they include people from your tribe as well as people from Vault 13. Speak with your Elder; she will tell you that you must melt down the facility's reactor.

Dick Richardson
Now go down the nearby stairs where you must solve the electric floor puzzle. Soon after passing the puzzle, you will come face to face with Dick Richardson, the "President of the United States," who will offer amazing amounts of historical and important information to you, mostly as a stalling tactic. You are allowed to kill him. In fact, this is the only way to obtain his access key.

The following options are available:
 * Use several super stimpaks on him, then wait using the function in the Pipboy for the after-effects to kill him (Enclave will not be alerted).
 * By planting an active explosive on him, then leaving to another section, he will either die or be weakened (Enclave will not be alerted).
 * Just kill him (Alerts the Enclave).
 * Sneak and shoot him. If he receives a critical hit, he will die (Enclave sometimes will be alerted, sometimes not).

Blowing the reactor
Go down the stairs northwest of the president's office (doing so after the Enclave are alerted by your assassination of the president will cease their hostility, even after you go back upstairs). Walk around until you find the reactor and the large mainframe computer that controls it. It is recommended that you have a plastic explosive. If you do, you can use it to blow up the large computer. If not, dynamite may suffice, or you may have to overload the reactor manually. There's also an option involving Speech/Charisma: you can attempt to convince Tom Murray (the head of the Atomic Energy Commission of the Enclave) to overload the reactor by threatening his life. This option will not be available, however, if you've already convinced Charles Curling to release his modified FEV into the Oil Rig's air supply: Tom Murray will still be alive, but his usual dialogue options will not be presented.

Facing the impossible
Once you've dealt with the reactor, go back upstairs, then find the emergency stairwell along the southern edge of that level (it was previously inaccessible, but is now the only way up). Those stairs will take you all the way up to the barracks near the entry hall. Soon, in the entry hall, you will face Frank Horrigan.

Granite and his team
As you leave the barracks, you'll see an Enclave team led by Granite. With a successful Speech check, you can convince them to fight in your corner. If you fail, you can easily re-try through the exact same dialog. Having them on your side, plus the presidential keycard, will improve success.

Using the keycard
The fight is extremely difficult unless you have the presidential keycard, which allows you to use the turrets against Horrigan (they will deal 15-40 damage). If you have the keycard, simply use the terminal in the SW corner of the boss room and activate the counter insurgency protocol (stay far away from Horrigan while approaching the terminal or he will initiate dialogue with you). This will make the turrets immediately start attacking Horrigan. If you also have the Enclave troops on your side, the odds will be very much in your favor.

Oil rig nuked
When Horrigan falls, you can talk to him or immediately finish him off. If you talk to him, he'll inform you that he's activated the Oil Rig's self-destruct mechanism, then die. At this point, all that remains is to escape from the Oil Rig by leaving the way you came in.

After the Chosen One escapes, there is a video clip showing the Oil Rig and its inhabitants destroyed by a huge explosion. After this, depending on the Chosen One's prior actions, there will be different endings.