I Put a Spell on You

 is a dual-sided quest in Fallout: New Vegas: it turns out a spy in Camp McCarran has been feeding information to the Legion about NCR troop movements. Depending on the side you help, you get to root them out and put an end to the problem, or make the NCR's life much more difficult!

Quick walkthrough (NCR)

 * Travel to Camp McCarran and speak to James Hsu or complete The Finger of Suspicion.
 * Speak to Captain Curtis (New Vegas) and then to Carrie Boyd.
 * Once you've got adequate information, head to entrance of the control tower.
 * Between midnight-2am the spy will wander over calmly (armed with a 9mm pistol).
 * Remove the disarming codes from the spy.
 * Head to James Hsu, and report the incident to him.
 * Head upstairs, directly over Hsu's room, and run for the monorail.
 * You must defuse the bomb. The quest will glitch and send the monorail to its death anyway even if you defuse it unless you follow steps noted below.

Detailed walkthrough (NCR)
(In order to complete this mission properly, and being able to also complete No, Not Much)

At some point, you will be asked to speak with Captain Ronald Curtis, who is located on the first floor of the Camp McCarran Terminal building. He will then ask you to speak to Carrie Boyd and Contreras. Talk to Boyd, and then ask her about break ins. She will say that there have been break-ins at the control tower every night. Ask her for an access key.

At this point, DO NOT go talk to Curtis, If you do, the monorail is doomed no matter what you do. Go out to the control tower, and wait there until roughly 1:30AM. It is helpful to create a save. You'll notice Curtis walking towards the tower at this point. It is very important that you remain hidden, otherwise he will confront you and ask why you're there. If you end the dialog successfully, he will then walk back into the base and you will need to wait until the next day.

If Curtis starts shooting at you, run outside to the tarmac, then run back to the tower without opening fire on him. Enter the tower and when you exit, Curtis will "forget" he tried to kill you moments earlier.

If Curtis doesn't see you, he will walk into the tower. Follow him inside, and stay hidden (use a Stealth Boy if your Sneaking ability are low and leave any companion outside) while you listen to him talking on the radio. The quest will update and tell you to disarm the monorail. Despite Curtis saying it will blow in minutes, there is actually no timer. If you have a high explosives skill or high science skill, you can choose to go directly to the monorail. Otherwise, you will have to pickpocket/retrieve an Activation Code item off of Curtis.

The fastest route will be to walk back outside the tower, and fast travel to McCarren. The door to the monorail is inside the terminal on the second floor, earlier in the day 2 armed guards block this door and will shoot you if you get to close.(On the opposite side from where the interrogation room is, if you've already done that).

Walk inside the monorail, and turn left. The bomb will be inside an air vent. Disarm the explosives using Science (>45), Explosives (>35) (magazines and/or Mentats are handy here) or the codes you "borrowed" from the spy's pockets. You may end up being transported to The Strip by the monorail - if that's the case just hop on and head back to McCarran.

Once the bomb is defused, if you have not already killed Curtis you may wish to return to the control tower and finish him off for the extra experience and his equipment. (note: you can get the equipment anyways, for Col. Hsu tells you he wants to arrest Curtis, if that one even lets himself getting arrested, what obviously isn't the case. You can loot his body in the tower after speaking to Hsu, all you'll miss is the extra-experience) Then return to Colonel Hsu: he'll reward you with some caps and a reputation boost. All done!

You can also complete the quest and gain the Experience Points with killing Curtis in the tower before hearing the radio conversation. This way, when you report it back to Hsu, he will be surprised and tell you Curtis could not be the spy as he was the leader of the investigation. After that, it will be revealed that there's a bomb on the train, the explosion of which you won't be able to prevent (I was able to get on the train and defuse the bomb, but for some reason still only had the option to tell Hsu that the monorail was destroyed). After the train exploded, you can report it back to Hsu, and then the quest ends, with you gaining nothing but the XP. This is a good solution if you run into the same problem as I did: there was no radio conversation, and the quest did not update itself, even after waiting hours at the bottom of the stairs.


 * Alternately, if you use the response "Hsu wont trust me enough to let me investigate." you can help The Legion to blow up the monorail. Fairly simple, follow the markers.

Behind the Scenes
Screamin' Jay Hawkins wrote the song I Put a Spell on You in 1956. Since then, many people have covered the song.

Bugs

 * After successfully entering and eavesdropping on the spy, leaving the watch tower causes any followers to stop following. They will wait in front of the tower until you talk to them and tell them to stop following, and then again to follow you.


 * It is possible to get the 100 cap reward for framing Private Crenshaw unlimited times, all is required to do this is to talk to him and tell him you found out who sabotaged the monorail and provide him with the speech check as evidence. Once you have done this repeat to keep getting the 100 cap reward. NOTE: this continues to work even after the quest has been completed.


 * After you investigate the late-night break-in, the game is not going to let you eavesdrop the conversation, and after a time, the spy will discover you. A possible workaround for this situation is this. Make a save outside the radio tower, after the spy enters it. If you hear the conversation when you go in, it has worked. However, the conversation will stop every time the spy has to give a reply. There are two ways to fix this. You can simply pause the game every time the spy has to reply, to continue the conversation. If this does not work, you must pickpocket one item from him for every reply. It is best to go with the weightless items. He is carrying 2 codes and 9 9mm cases, which you can still steal with a low stealth skill (20). Steal any of those items in any order you wish every time he needs to give a reply, but only steal one item, because the 9mm pistol and ammo are harder to steal.


 * There's a bug which prevents you from saving the monorail, even if you get on the monorail and disarm the bomb, the monorail still blows up (still unfixed on the Xbox 360 even after installing the 1.01, and on the PC after the second update patch, confirmed on the PS3 version). Hence, if the player wishes to access the strip easily through the monorail, they should do so before initiating this quest OR the portion of the quest initiated where the player has to save the monorail, until the bug IS fixed.


 * If you do defuse the bomb though, Radio New Vegas will report you foiled the plot.


 * If you end this quest without saving the monorail, Hsu will not grant you the reinforcements for Bitter Springs, thus making the Quest No, Not Much unsolvable. (Work around: Reaching 'Liked' status with the NCR allows Hsu to send the reinforcements before completing this quest.)


 * If you slide on the rail to the door of monorail after killing a spy, you will be able to enter it as usual, and defuse the bomb. Also, you will get the caps for it.  Weirdly, though, it does not affect the dialogue, and might also not affect the explosion of the train (needs verification).


 * If you quicksave just before Curtis enters the tower and then reload, Curtis remains standing exactly where he was when you saved, but is invisible and will not respond to either speech or pickpocket attempts. A partial fix is to head back into the terminal building and wait till daylight hours. Curtis will reappear and you can now interact with him, but he will remain in position, thus never heading into the tower and making the quest unsolvable.


 * If you decide to leave the tower when you're suppose to listen to the conversation and teleport yourself to some other place and wait there (3 hours?), Curtis will appear and ask you what you are doing there (in another language with normal sub). If You can end the conversation well, he will travel back to the camp. The only problem, you wont be able to listen to the conversation anymore.


 * If you're on bad terms with the NCR and initiate this quest with as a Caesar's Legion supporter, then get on good terms and speak to the Colonel about completing the quest for the NCR. Then the explosives in the trash can become non-existent and the Control Tower becomes vacant forever.


 * Sometimes no matter what time or day your there the spy will never show

Fixes
If you've killed the spy and already spoken to Hsu about it, but don't want the monorail to blow up, you can follow the steps here to create a mini-mod which would allow you to stop the explosion and complete the quest (not working to everyone, although following this instructions, it may work).