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 Empty. He will say that a visitor (presumed to be Father Elijah) attacked Dr. 0 and Dr. 8, destroying 8's voice chip, and escaping from the Big Empty, while also managing to take control of the trains in The Big Empty and crashed them all, making an exit for themselves. The player must then ask Dr. 0 about the attack.