The Hiss threat might have been pushed back at the end of Control, but it still persists through the game’s DLC expansions. The first story based addition is The Foundation, an addition that gives Control's combat a more environmental focus, while providing a look at the origin of the Bureau of Control's use of a building that might even be alive.
Control picks up where the main story left off to tie up a loose thread: What happened to Marshall, the Bureau's head of operations. During the vanilla game, Marshall disappeared during the Hiss crisis to deal with some other, unknown problem. Turns out, she headed down to the Foundation--the lowest point in the shifting office building known as the Oldest House, and a place you only visited briefly during the course of stopping the Hiss from taking over the Bureau.
The Foundation is actually a huge set of caves running beneath the Oldest House, providing a new location to explore for the duration of the DLC, which runs five or six hours if you're willing to track down all its secrets. At the center of the cave system is a strange monolith called the Nail, which has been damaged and needs repair. The Nail actually seems to be a piece of the Astral Plane that exists on Earth, and which might give the Oldest House its power. With it damaged, though, you have a major problem: The Astral Plane and the Oldest House are beginning to merge, and if the process isn't stopped, both dimensions could be destroyed.
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