When Visitors Attack!

When Visitors Attack! is a side quest in the Fallout: New Vegas add-on Old World Blues.

Quick walkthrough

 * 1) Speak to O|Dr. 0.
 * 2) Speak to Dr. Klein.

Detailed walkthrough
Ask Dr. Klein about previous visitors to the Big MT. He will say that a visitor (presumed to be Father Elijah) attacked 0|Dr. 0 and 8|Dr. 8, destroying 8's voice chip, and escaped from the Big MT, while also managing to take control of the trains in The Big Empty, crashing them to make his exit. This caused the Big MT east tunnel to be completely blocked off. The player must then ask Dr. 0 about the attack.