I Put a Spell on You

 is a quest in Fallout: New Vegas: it turns out a spy in Camp McCarran has been feeding information to the Legion about NCR troop movements. You need to root them out and put an end to the problem!

Quick walkthrough
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-1am he'll wander over calmly. He's armed with a 9mm pistol in good condition. Bringing a buddy never hurts (I brought Boone, after I put 4 shots from my silenced .22 [i was trying to be covert], Boone put him down with a single shot from his hunting rifle).

Strip the spy of his posessions, 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. It isn't supposed to be stopped. If you do stop the train, the quest glitches.

Detailed walkthrough
First, you need to track down and talk to Lt. Boyd and one of the members of the First Recon Sharpshooter's group. Boyd will be somewhere in the main terminal building, rather than the concourse. If you already completed the Three-Card Bounty quest, the First Recon will probably be some way north of Helios One, possibly on their way to Camp Forlorn Hope, or at the camp already. If in doubt, follow the markers.

Your "contact" in the First Recon will tell you that the lights have been coming on in the Control Tower every night at 1am. Now you know where to start. Head back to McCarran and make sure you got the key for the tower from Boyd. Hacking the access panel is non-essential, and doesn't really give you much to go on, beyond some slightly suspicious access notes. Stake out the tower until the spy arrives, then follow him in and hide at the bottom of the stairs. If you have any Stealth Boys handy, now might be a good time to use one if your Sneak isn't up to the task.

It is essential that you are not discovered before the eavesdropping is completed. If you are, the spy will initiate conversation with you and ask why you are in the tower: no matter your excuse, he will attack you. Killing him at this phase will cause the quest to fail.

Warnings aside, stay at the bottom of the stairs and eavesdrop on the conversation until the objective is marked as completed. If you're stealthy enough, or your Stealth Boy is still active/you feel like using another one, sneak up and "borrow" the spy's belongings from his pockets; you want the Activation Codes primarily. Now make a beeline for the Monorail Station - don't go to Hsu as you may not be able to tell him anything at this stage. Get on the train and check the vent near the door on your left. There are explosives hidden inside!

Disarm the explosives using Science (55?), Explosives (45?) (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!