Mesmetron

The  mesmetron is a prototype stun gun in Fallout 3.

Background
The mesmetron is an experimental pre-war "non-lethal pacification" weapon that was being tested in the months immediately preceding the Great War. It operates by sending a signal pulse into a human target's brain that scrambles higher-level mental functions, leaving the target extremely susceptible to suggestion. Still in its testing phase, the weapon was found to have the potential to trigger two undesirable side-effects: extremely increased aggression, or spontaneous cranial eruption.

Characteristics
When used on the specified quest NPCs, it puts them into a hypnotic stupor, allowing the player to rob and enslave them (by putting a slave collar on them) and send them to Paradise Falls. It also appears to work on many other NPCs (aside from quest related ones), but can simply send them into a berserk frenzy in which they become hostile towards the player and every other NPC in the area. Occasionally, victims will run several steps before their heads explode.

It uses mesmetron power cells as ammo. You start out with 50 mesmetron power cells. Grouse will also sell you additional power cells, but at the fairly high price of 200 caps for 10 power cells. This gives you the opportunity to make an additional 2500 caps every time you purchase more power cells (provided you successfully capture all 10 slaves). An additional 71 mesmetron power cells can also be found in the Underground Lab in the ''Point Lookout add-on.

The mesmetron can fire a total of 2475 waves, or 495 full cells, before breaking.

Using the Mesmetron
As previously mentioned, using the mesmetron on an NPC can have 3 different outcomes. These effects vary and are generally unreliable, so it is advised to save before using this weapon on anyone important. Due to the fact that using the Mesmetron is considered assault, it is advised to "mezz" someone while not detected (stealth boys and the Chinese stealth armor will come in handy here). Be aware that mezzing anyone who is already wearing a slave collar will cause the collar to malfunction and explode, killing the victim. Hostile NPCs who were mezzed can also be turned friendly by being freed by the player (high Science skill required). An analysis of the Mesmetron's effects:

Mesmerize/Stun (50% Probability)
This first effect stuns the victim for a few seconds, during which he or she will stand still and their heads sway from side to side. When the victim is stunned, it is possible to engage in conversation with the victim. Note that some characters cannot be stunned, in which case the probability of the Frenzy effect is raised to 80%.

While in conversation, you will first have 3 options. The first option convinces the character to allow you to take his/her things (by either telling them it's a magic trick, that there is a thief on the loose, that you think they stole something from you, or by just telling them to give you their things) which will open up the victim's entire inventory (even the items he or she is wearing/using) and allow you to take or even add any item. The second option slips a slave collar on the character (instantly netting the player negative Karma regardless of who the victim is). The final option simply tells the victim to "walk it off"; this will exit the conversation and make the character resume what they were doing (presuming you haven't made any onlookers hostile).

The second set of dialogue options (unlocked by slipping a collar onto the victim) is quite simple. The first option is the "cruel" way to inform the victim that he is now a slave ("...your head will explode!"). The second option once again allows the player to check the slave's inventory, and the third option is the slightly nicer way to inform the victim of his/her "new occupation" ("...nothing personal").

If the character is not talked to before the stun wears off, he or she will become hostile.

Frenzy (30% Probability)
This second effect causes the victim to go into a state of frenzy, attacking anyone nearby (most likely the player), and causing all other characters to turn on them. The onlookers may also become hostile to the player if you are seen stunning the victim. This method can also be a reliable way to get rid of victims without attacking them or getting negative Karma. You can die from this, especially if one of your enemies has any high-damaging weapons.

Spontaneous Cranial Eruption (20% Probability)
This final effect of the Mesmetron may not be the most useful, but is definitely the most spectacular. When victims are affected by the mesmetron in this way, they will become hostile (and either run, stand still, or attack) for just a moment before their heads literally explode. This effect will instantly get the player negative Karma (if the victim was good/neutral) and make any nearby NPCs who were not hostile to the victim become hostile to the player.

Variants

 * Microwave emitter (Point Lookout add-on) - A similar weapon designed for killing targets as opposed to stunning them, found in the St. Aubin Medical Facility.

Location
The mesmetron is given to you by Grouse, the Paradise Falls entrance guard, when you start the quest Strictly Business.

If you don't want to take the quest, you can obtain the Mesmetron by killing Grouse and looting his corpse. You have to kill him to obtain the Mesmetron, as it cannot be pick-pocketed. If you do this, you cannot get a slave collar and thus can't enslave NPCs. You can still use the mesmetron to frenzy or stun and rob NPCs.

Related quests

 * Strictly Business

Bugs

 * Attempting to remove a collar from an NPC enslaved by the Lone Wanderer (other than Susan Lancaster, Flak, Arkansas, and Red) can result in a crash to desktop, depending on the NPC's actions immediately prior to being talked to. In general, crashes occur less frequently the shorter the time is between enslavement and collar removal, and also are less likely if the NPC is not on their usual patrol/movement route.
 * Removing a collar from an NPC who was mezzed while in combat may cause them to die once the collar is removed. This seems to happen more often (but not only) when the NPC was stripped of their armor and/or at low health before being mezzed. The NPC's death does not appear to be attributed to the player and will not give XP.
 * "Mezzing" characters who are seated, leaning on a wall or lying down may cause them to spin rapidly in place before their heads explode.
 * When you untie a captive and then use Mesmetron while he or she is getting up, the person will be stunned and not be talkable. This is annoying because when the person is unstunned, the person will appear like the person is tied up and you can't talk to the captive anymore.

Gallery
Гипнотрон