I Put a Spell on You

 is a quest in Fallout: New Vegas.

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.