Fallout: New Vegas/I Put a Spell on You

This quest is obtained from two places, and involves intelligence leaks at Camp McCarran. Despite two different choices with this quest, it is not possible to double-cross either faction - once you receive the quest, you are stuck along the path to completion.

From Hsu
When you speak with Colonel Hsu about the intelligence leaks, the quest will become active. However, if you are hostile with the NCR, you will need to pass a speech check to continue with this quest line. He will refer you to Captain Ronald Curtis, located on the first floor of the building, on the western wing. In turn, Curtis will refer you to Lt. Boyd.

Lt. Boyd will provide information about break-ins to the control tower, occurring during the night. If you are on good terms with the NCR, she also has a small task where you can extract information from a prisoner (requires speech or intelligence, or otherwise a long combat process) who states that the informant is an officer.

To catch the spy, wait outside the control tower, hidden from too much view. He should arrive sometime around 1AM if you stake out the building around midnight. Once he enters the control room, enter and listen in to what's being leaked. He will mention that the bomb is in the monorail, ready to detonate at any moment.

To disarm the bomb, you need an explosives skill of 35, science of 45, or the activation code from Curtis. To safely reach the monorail, you need to be at least Liked by the NCR, be wearing NCR faction armor. In both cases, you can safely kill Curtis if you provoke him into attacking you (e.g. approaching him in the tower).

Additional notes

 * If you inform Captain Curtis about potential leads, he will try ambushing you in the tower.
 * There is a time limit on this mission, although there are some cases where the timer might not start.
 * If you have Boone as a companion, saving the monorail helps unlock his quest.

From Vulpes
After completing ../The Finger of Suspicion/, you can continue speaking with Vulpes to find out about the informant at Camp McCarran. Captain Ronald Curtis is your contact, and is found on the western wing of the first floor. However, if the NCR labelled you as a terrorist because of your progress in the main quest, Curtis will attack and cause you to fail the quest.

He will give you a few tasks:
 * 1) Speak with Colonel Hsu and state you want to investigate the leak.  If you are on bad terms with the NCR, you need to pass a speech check to succeed.  Regardless of the result, return to Curtis and report.
 * 2) Collect the materials in the main terminal building, located in a garbage can.  This includes the bomb, and components for a homemade explosive.   This step commits you to bombing the monorail.
 * 3) Sneak into the Monorail.  This requires a reputation of at least Liked with the NCR or otherwise wear their armor.  There are also times where the solders are not at the monorail gate, allowing you to sneak past.
 * 4) Plant the explosive within the vent, located on the back of the monorail. When you exit, it will detonate.
 * 5) If he is not dead already, kill Private Crenshaw.  You will also need to plant the components in his footlocker.

Return to Colonel Hsu, and tell him you know who bombed the monorail. You can succeed with a speech check, or by telling him about the evidence. Return to Curtis and let him know he is in the clear.

Additional notes

 * If Hsu attacks you because of a failed speech check or through known dialog options, Curtis will not appreciate the result.
 * If you kill Curtis, the mission will fail but you won't be able to stop the bombing.