The Darkwaters General Store was founded by Killian Darkwater sometime after his grandfather founded Junktown.
Killian will trade with the Vault Dweller and there are items located around the store which can be taken and a safe to be cracked. Killian's locked safe when picked grants an additional 500 XP, and over 2,000 caps.
While hanging around the store, a "shady gunslinger" walks in and tries to assassinate Killian (even though he employs three Junktown guards). After surviving this attempt on his life Killian will initiate the quest to get Gizmo's confession, as well as give the player character two unique quest items.
Helping defend Killian from the hired assassin's attack also grants 400 XP. Helping defeat Kenji without taking any damage grants a bonus of 250 (making 650 XP altogether).