-the theme of the DLC is "home" - in regards to the Courier's, Ulysses and ED-E's as well as the nature of The Divide.
-some time before FNV, Courier Six founds a settlement in the Divide, and this is where Ulysses made his new home after the Legion took his over - Dry Wells (?) where the Twisted Hairs tribe (I assume that means dreadlocks or something seeing as he has them) were assimilated into the Legion.
-The Divide was a community that was important enough that it was like a proto-Hoover Dam; that it could connect both the West (NCR expansion) and East (Legion expansion) and bridge them together, possibly reconnect both sides of the US.
-Pre-war Divide was a military base/community where they had nuke silos underground.
-For one reason or another, the Courier accidentally (?) brings in package from NCR territory that has codes to all the nukes underneath the Divide and they get set off underground, leading to what you see in LR - cracked ground, mutated subterranean creatures, the Marked Men, etc.
-Now rather than the Divide be the focus in the Mojave region (it was supposed to be a vital trade route prior to the incident), now the focus lies on Hoover Dam.
-Ulysses is the only other survivor of the incident realizes that the NCR/the West cannot help reunify the US and plans to nuke the Long 15 to cut off supply lanes from the NCR to the Mojave, effectively ending NCR expansion plans. (brings up the subject of NCR being 'diseased', cannot keep itself together, etc.)
-you learn more about ED-E's mission to Navarro and how it came to be; also that Ulysses needed the ED-E you find in Hopesville to help him prep the nukes to launch.