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)
Follow the markers on your compass to where you need to go. Speaking to some local NCR named troops as necessary to gain information.

Once you've got information adequate to hunting down the spy in the tower, head out that way. Around midnight-2am he'll wander over calmly. He's armed with a 9mm pistol in good condition. Bringing a buddy never hurts.

Strip the spy of his possessions, or at least the disarming codes. Head to Col. Hsu, and report the incident to him. Head upstairs directly over Col. Hsu's room, and run for that monorail. Depending on who you talk to, the train will either leave before you can board it, or you will be able to successfully defuse the bomb. This is not a bug or a glitch. If you do stop the train even though you aren't supposed to, the quest glitches. You can stop it if you follow the steps below.

Detailed walkthrough (NCR)
At some point, you will be asked to speak with Captain Ronald Curtis, who is located on the first floor of the terminal at Camp McCarran. He will then ask you to speak to 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. 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 he doesn't see you, he will walk into the tower. Follow him inside, and stay hidden 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 (70 with a magazine or 80), 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. Once the bomb is defused, head back to Col. Hsu and inform him of the situation. 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. Head downstairs and talk to 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. 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, which's expolsion you won't be able to prevent. 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.

Behind the Scenes
The name of the quest could be a reference to the 1956 Screamin Jay Hawkins song of the same name.

Bugs
In the PC, PS3 and Xbox 360 versions of the game 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, unconfirmed on the PS3 version).

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).