The quest is given to the player in the Temple of the Union by Hannibal Hamlin.
You will be asked to find out if the Lincoln Memorial to see if it is still occupied by Super Mutants. Also you will be asked to escort Caleb Smith to obtain a picture of the Lincoln Memorial in the Museum of American History.
After scouting the Lincoln Memorial, the player is required (as an objective) to clear the Memorial of all enemies, specifically the slavers currently occupying the Memorial. If the player enters the Memorial, the slavers will immediately become hostile and shoot the player, regardless of whether the player has the "Head of State" quest. Once the Memorial has been cleared of all slavers, the player can return to the Temple of the Union.
When the player reports back to Hannibal Hamlin that there are no Super Mutants in the Lincoln Memorial, Hannibal will gather up the runaway slaves and head for the Memorial.
In the event that the slavers still occupy the Lincoln Memorial when Hannibal Hamlin and the runaway slaves arrive, they will ask the player to help them kill the slavers. If the player does not tell Hannibal to wait while you check it out, then Hannibal will lead the runaway slaves in an assault on the Lincoln Memorial. Hannibal is usually killed, which will terminate the quest. However, if the player tells Hannibal he/she will "investigate" the Memorial, Hannibal will wait. This method will allow the player to go into the Memorial and kill the slavers occupying the Memorial. Then, when the player reports back to Hannibal, there will be no one to kill Hannibal when he leads the charge and the player can complete the quest