Sometime before the Great War, a prototype plasma pistol of what later became the Enclave plasma pistol, was stolen from the federal government by an organization called Quaere Verum. The pistol was later taken by aliens who, most likely unknowingly brought up both the safe that contained the gun and the computer terminal required to remotely unlock the safe.
The weapon acts very similarly to the plasma pistol it formed the basis for, barring several prototype features. First and foremost, this weapon does more than three times as much damage as the regular plasma pistol, and has a higher critical multiplier (3x as opposed to 2x). However, it uses two energy cells to power each shot, and weighs twice as much as the normal, almost as much as an R91 assault rifle.
The weapon is found in the cargo hold area of Mothership Zeta. In one area is a Reid Underwood's terminal (hard) which can be hacked to unlock a safe, to get to the terminal go to the Research lab door go north then a bit east you should see a group of terminals, it's the only one that isn't broken. The safe itself is found in another area among several piles of rubble, next to a machine that is dropping items into a pit. Inside the safe is the weapon, a note detailing the weapon, and energy cells (amount may vary due to the Scroungerperk).
The Cargo hold becomes inaccessible after teleporting to the Bridge, making it impossible to obtain the weapon afterwards.
xbox360ps3pc While using V.A.T.S., selecting a target causes the ammunition counter to drop by one, not two. The bolt will still use two energy cells, but this may force an unexpected reload sequence. [verified]
xbox360 Sometimes Paladin Hoss will unexpectedly show up (even though he's not supposed to) and take the pistol after you've unlocked the safe. [verified]