Dealing with Contreras

 is an unmarked quest in Fallout: New Vegas.

Quick walkthrough
There are three ways to complete this side quest, but only two are guaranteed to reward you with the unique weapon This Machine. You can:


 * 1) Side with Lt. Boyd and gain NCR fame and This Machine. This is the quickest option, but you will lose Contreras as a merchant. Either speak to Lt. Boyd to start the quest, or simply hack Contreras's computer (Science 50) in the Supply Shack. Download the weapons manifest and report back to Lt. Carrie Boyd to complete the quest and receive This Machine/
 * 2) Side with Sgt. Contreras and expand his inventory and receive This Machine. Speak with Contreras about his supply. He wants you to run several errands for him. You will need to be level 12 or above with a high Barter/Speech skill (60+) to complete Contreras's errands. Depending on your stealth skill, you may be forced to incur a reputation loss with the NCR to complete Contreras's last mission. Contreras will reward you for completing the quest by offering a much wider selection of weapons and ammo, as well as by giving you This Machine.
 * 3) Side with Keller. To even meet with Keller, you need to complete Contreras's first two errands. When he gives you the third mission, you may encounter the option of siding with Keller. Doing so will complete the quest, but depending on your actions you may lose access to Contreras as a merchant and/or to his special stock. Neither Keller nor Contreras will reward you with This Machine for completing the quest in Keller's favour.

Siding with Lt. Boyd
You can start this quest by either speaking to Lt. Boyd in the Camp McCarran terminal or by proceeding directly to Sgt. Contreras's terminal in the Camp McCarran Supply Shack and hacking it (Science 50). The Supply Shack is at the back of the terminal building via the Camp McCarran Concourse. Enter the terminal, go up the escalators, down the hallway to the concourse. When in the concourse bear left to a down ramp, and bear right to the door of the supply shack. Contreras's terminal will be on the upper level of the supply shack.

Downloading the weapons manifest and returning it to Lt. Boyd will complete the quest. She will reward you with This Machine and you will receive NCR fame. By turning in Sgt. Contreras, you will remove him from the game and you will lose access to him as a merchant. Since he is one of the best weapon merchants in the game, and since the quest reward (the unique This Machine) is awarded to you regardless of whom you support, you should think carefully before siding with Lt. Boyd.

You can also complete this quest at any time if you have sided with Contreras or even Keller. If you downloaded Contreras's manifest, or ran his errands and passed the Medicine check where you notice that his "medical supplies" could be used to make chems, you can either of these to Lt. Boyd at any time to complete the quest and receive This Machine. Sgt. Contreras will wind up in a prison cell, where he will still act as a merchant though he won't restock like other merchants.

Siding with Sgt. Contreras
You must be level 12 to start this quest. You can start it by finding Sgt. Contreras in the Camp McCarran Supply Shack. The Supply Shack is at the back of the terminal building via the Camp McCarran Concourse. Enter the terminal, go up the escalators, down the hallway to the concourse. When in the concourse bear left to a down ramp, and bear right to the door of the supply shack.

Speak to Sgt. Contreras about his supply, and you'll have to pass a Speech/Barter check of 50. Contreras will then ask you to speak to Isaac outside of Gun Runners.

Isaac should be located near or at the reloading bench to the right of the Vendortron at the Gun Runners storefront. (If Isaac isn't there, he may be in his house to the east of Gun Runners. If he isn't there either, check the Bugs section for this quest.) Speak to Isaac and you'll either have to pass a Speech/Barter check of 80 OR you can give him 100 pieces of junk metal (tin cans, bent tin cans, metal pots, pots, pans and scrap metal). Once you've done this, you can return to Contreras.

Contreras's next errand will be to obtain some medical supplies from Blake in the Crimson Caravan compound. With high-enough Science skill you can observe to Contreras that the supplies he's acquiring could be used to make chems, or you can point the same thing out to Blake. However, doing so to either man will have no effect on the quest whatsoever.Once you have the supplies, return to Contreras. He'll tell you he needs a "couple days" before he has another mission for you. Waiting for 24 then 18 hours seems to be sufficient to trigger the nest quest.

Contreras will then ask you to deliver the medical supplies to a man named Price in Miguel's Pawn Shop. He'll also give you a passphrase to speak to Price, and he'll tell you what passphrase Price should say to you in return. When you enter the pawn shop, though, Price is nowhere to be found. Instead, speak the passphrase to Keller. Keller will not know the correct response. The quest can be resolved in Contreras's favour by:


 * 1) Before saying anything else to Keller, you can exit dialogue and return to Contreras to inform him of Price's absence/Keller's ignorance of the passphrase. Regardless of which you tell him, Contreras will tell you flat-out to kill Keller and he'll take back the medical supplies (which you wouldn't otherwise be able to remove from your inventory as they're marked as a quest item). When you return to Keller, you can enter into dialogue, but your only option is to say "Goodbye." You must kill him to complete the quest.
 * 2) With a Perception AND and Intelligence score of 7, you can observe to Keller that he didn't know the correct passphrase, but this just forces Keller to expose his identity slightly earlier. He also won't be able to confiscate the medical supplies from you.
 * 3) The other option is simply to deliver the package to Keller instead. When you do so, Keller will confiscate the supplies, expose himself as an undercover NCR ranger attempting to shut down Contreras's operation, and threaten to place you under arrest. If you want access to Contreras's inventory, or you want This Machine, your only option is to kill Keller. You can do this at virtually any point in the conversation, as there will almost always be a [Draw Weapon] option during all dialogue with Keller although this . You are also free to exit dialogue and kill him, as he'll just stay friendly if you don't provoke him. Keller can be difficult to kill as he is an NCR citizen, and killing him can incur a reputation loss with the NCR. However, it's more than possible to kill Keller before Miguel notices with a high sneak skill and a silenced weapon. Doing so and speaking to Contreras will successfully complete the quest (though Contreras's dialogue may not accurately reflect the course of action you chose).

If you've settled things in Contreras's favour, he'll award you with This Machine and the next time his inventory restocks he'll have a wider selection of items.

Siding with Keller
Remember that by siding with Keller you forfeit This Machine, and depending on your actions you may lose access to Contreras as a merchant. To complete the quest in Keller's favour, you must have already completed Contreras's first two missions. When he sends you to speak to Price in Miguel's Pawn Shop, Keller will be there instead. The most direct way to complete the quest in Keller's favour is to give him the medical supplies.

Once you do this, he'll threaten to arrest you. You must claim you're innocent, and blame Contreras. Keller will be pleased that his suspicions of Contreras have been confirmed. You then have to pass a Speech check of 60 claiming that you didn't know what you were carrying. Keller will believe you and let you go free, but promise to arrest Contreras. At this point, you can leave, which will get Contreras arrested and you will lose access to his expanded inventory. There is very little reason to do this.

You can also bribe Keller 250 caps to throw his lot in with Contreras. The attempt will fail, but you can offer Keller 500 instead and convince him to offer Contreras a chance to inform on his clients. Keller will accept and ask you to bring the deal to Contreras. Return to Contreras and inform him that Price is being held by the NCR and a Ranger has taken his place. If you were forced to give Keller the medical supplies, Contreras will demand that you kill Keller.

You could return and kill Keller, in which case consult the "Siding with Contreras" section above. If you are determined to side with Keller, then with a Speech of 80 or a Barter of 70 you can convince Contreras to cut a deal with Keller. Contreras will ask you to speak to Keller once again, and you must pass a Speech check of 80 to get Keller on board. (Be warned that if your Speech is less than 80 the option won't even appear in dialogue and you'll be forced to attack Keller.) Convincing Keller will complete the quest, awarding you 80 XP but not This Machine. Contreras will still deal with you, though it's unclear if you ever gain access to his special stock.

Videos
BtyzCnn17QM

Bugs

 * Sometimes your contact for the third errand will not show up.
 * The responses for the third errand are all messed up if you choose to kill the officer first.
 * After the third errand he will disappear and won't be where you previously talked to him.
 * When you first speak to Contreras there will be no option to start the quest, just to browse his inventory and to leave.
 * Isaac may disappear from the game.  Isaac may get stuck in his house and stop appearing near the Vendortron.  If this happens use something with low damage to damage him, then holster your weapon and you can talk to him.  Go look for him in his house (I believe E-SE of the Vendortron, check local map).
 * Isaac may also be killed by Fiends while going to and from his home and the Gun Runner's kiosk.
 * After delivering the medical supplies/chems, when Contreras tells you to 'come back in a few days', you are given the dialogue option to say "So where was I supposed to meet Price, again?" even though you haven't been given that part of the quest yet.
 * Third Errand: If you pass the 70 barter check with Agent Keller before convincing Contreras to work with the Ranger, Keller will tell you to go back to Contreras and offer him the deal, but Contreras will have no more dialogue options, and the player will be locked out of completing the quest and receiving "This Machine".
 * Workaround: If you choose to just kill Keller at this point, the dialogue option to get "This Machine" should still become available. He can be killed stealthily without gaining NCR infamy.  Talking to Boyd does nothing, as there is no longer an option to rat Contreras out after making the 70 barter check deal with Keller.
 * Even with 7 or higher perception, the speech challenge which results in you helping Keller arrest Contreras will always fail, since the perception check is run on Keller instead of the player.
 * The dialog when you give Contreras up freezes the game.
 * After the quest is completed and Contreras is locked up in one of the cells, it is possible for you to initiate dialog with him (via 3rd-person view) and browse his wares. However, the game will freeze upon exiting the buy/sell screen.
 * Contreras after completing the third task and obtaining "This Machine" will still sell to you, but will not give you the goods you purchase, but will gladly take your caps.
 * After he turns over This Machine and allows you to purchase from him, any attempt to trade with Contreras results in the game freezing upon exiting the bartering screen... making you wish you just turned him in to Lt. Boyd.
 * Sometimes when given the medical supplies, they can be sold and lost forever; making it unable to get This Machine.
 * The medical supplies can sometimes stay in players inventory even after delivering them to Contreras.
 * If Contreras sends the player back to kill Keller, and the player does, upon returning to Contreras the dialogue prompt will read, "I convinced the Ranger to work with you." Regardless, the task will still complete, and Contreras will still reward the player with "This Machine".
 * Contreras' claims that if the player completes his last task, he will give the player "full access to my stash. At a discount, even", however there is no discount from Contreras ever offered, no matter how you resolve things.
 * When attempting to sell Contreras your equipment, upon exiting the buy/sell screen, the game with crash and require a hard reboot.
 * Lt. Boyd is apparently part of the First Recon unit. If you complete the headhunting quest given by Major Dhatri before you complete this quest, she will leave with the rest of the unit and not be available.