First, the player needs to travel to the Mariposa Military Base. It's located to the extreme northwest of the map. Its location can be obtained from the computer just beyond the airlock in the Master's Vault. Harry will also instantly take the player to the Lieutenant.
To enter the base, the player needs to get through the mutants and the security door. Mutants outside or inside won't react to party members.
Combat characters can simply kill them all and loot the holodisk from the guard by the entrance (the one in the black uniform). If inclined towards teamplay, it's possible to persuade the Elders at Lost Hills to send three crack assault Paladins to Mariposa by first completing Maxson's request to scout the base, then asking for help in attacking it. Each wears a suit of T-51b power armor, and they are armed with a CZ-53 personal minigun, an L30 Gatling laser, and a BigBazooka rocket launcher. They will not enter the base, however.
Stealth characters can sneak through and either pick the lock on the door or steal the mutant's holotape. Katja will crack the lock on the door if the player uses it and she is in the party.
Diplomatic characters can bring robes with them and pick the lock on the door the same way. A risky way to enter the base is to talk to the mutants and agree to meet the Lieutenant.
All characters can use a radio to bluff the guards into leaving, by pretending to be a patrol that came under attack. The guards will wander off, leaving just the door guard. This is worth 1500 xp.
The inside of the base will be put on alert if any combat lasts more than two rounds. This will turn on the yellow forcefields and change the positions of a handful of super mutants on levels 1 through 3. All mutants will also attack the player on sight.
Yellow forcefields can be toggled by using the computer in the level 1 room to the south-east (the one that is not described as a Mach IV). Using it will eventually result in the player breaking security systems (800 xp for completing one of the only Gambling checks in the game). Using Science afterwards theoretically toggles the forcefields across the base, but a bug prevents the variable from being set. Instead, the player can also use the radio on the computer and after passing a Repair check use it to toggle forcefields on and off. Finally, using Repair on active yellow forcefields disables them temporarily and explosives destroy them permanenty.
Another optional computer on level one is located near the elevator. Using Science on it allows for the player to fiddle with the Robobrain settings across the base. Setting pest control to all, small, or large will kill three mutants in the armory on the level below (useless, as all box containers across the second and third levels are empty).
The goal lies on the third level. The second houses the mutant barracks and is actually entirely optional, save for major loot in the lockers and medical supplies in the med bay (after killing the mutant with a very small cranium, which can be done without alerting others if the player is quick enough).
To safely enter the cell block as a stealthy or diplomatic character (combat characters would be knee deep in the dead at this point, blowing their way through), robes are required. Otherwise, the character will be attacked or taken to the Lieutenant.
The goal lies on level four. The control room has a number of technicians around, who will not attack. Killing them triggers the alarm regardless of the way it was accomplished in. The Lieutenant is in his private quarters. The player can eavesdrop on his conversation with VanHagan by moving just one hex around the corner. Listening to it will earn 1000 xp.
The Lou is an optional encounter (less so if the player was taken to him). He will attack along with his coterie if the player approaches him and doesn't surrender. If the player was captured by a mutant, the Lieutenant will first interrogate them, demanding the location of Vault 13. Surrendering it leads to the Master's ending. Otherwise, the Lieutenant will torture the player, hitting three times in a scripted sequence (taking down most of the hit points, but cannot actually kill the player character), then have them stripped of their gear and locked up in the cell block (party members are moved alongside the player, as the location was apparently completed before the mechanics were implemented and thus no special script was added). The player can pick the lock on the door, then the game continues as normal. The inventory is stashed in the locker in the guard room near the elevator on level 3.
To destroy the base, use the control computer to activate the self-destruct. This can be accomplished by blowing up the computer with explosives (300 second countdown with alert active). The computer can also be hacked by using Science or the security card swiped off the technicians. The options available after hacking the terminal are mislabeled and represent, respectively: 60s, 270s, setting off the alarm, 300s, another 300s, and finally, a 1s delay on the self-destruct (which is bad).
Activate the self-destruct, then leave. All countdowns are silent. To properly exit the location, the player needs to step on the red exit grid - standing on the surface map will count as perishing along with the base.
