After Elder McNamara is ousted and Head Paladin Hardin becomes the new Elder, you are given the opportunity to join the Brotherhood of Steel through a heroic act. Hardin already has a task in mind; to wipe out the Van Graffs as a show of loyalty to the Brotherhood.
Due to well-equipped guards, and a mandatory pat-down upon entering the Silver Rush (the Van Graff base of operations) killing all of the Van Graffs can be a difficult task. However there are multiple ways to kill them.
Killing Simon the door guard allows the player to enter the Silver Rush without surrendering their weapons.
Weapons can be obtained inside the store though either theft or by buying them.
During Birds of a Feather the fourth and final customer will attempt to blow the store up and, if allowed in, will result in the death of all Van Graffs, with the exception of Simon. However, this also results in the destruction of much of the merchandise as well.
After eliminating the inhabitants of the Silver Rush, return to the Hidden Valley Bunker. Once there, inform Hardin that you have completed his task, and he will reward you with full membership into the Brotherhood of Steel, rewarding the player with a suit of T-45d power armor and a T-45d power helmet (a formal ceremony would apparently be inappropriate). In addition, the player receives the Power Armor Training perk, if they do not have it yet.
This quest is the alternative to Eyesight to the Blind, which is McNamara's loyalty quest. You can only do the quest specific to the Elder you chose to support, which is decided once Still in the Dark is completed.
If you have completed Eyesight to the Blind, this quest isn't marked as failed unless you later destroy the Brotherhood of Steel or kill Head Paladin Hardin.
This quest can also parallel with Cass'Heartache by the Number quest, which could involve the elimination of the Van Graffs, if you've taken the violent path. If you have completed this path, and then receive this quest and it still says to kill the Van Graffs and their men, even though they are all dead.