Save Killian's life

 is a side quest in the original Fallout.

Walkthrough
In the Crash House area of Junktown, go to the Darkwaters General Store. Talk to Killian Darkwater and head for the exit. As you do so, a man named Kenji should walk past you and attempt to assassinate Killian. If you help Killian kill Kenji you will open up the Get Gizmo's confession quest line.

Note that you must not have been hired by Gizmo in order for Kenji's attempt to occur, presumably since Gizmo would have only hired one assassin at a time for whatever reason.

Reward
400 XP and +2 or +3 Karma depending on whether or not you delivered the final shot to Kenji.