Keeping in line with Control's mysterious atmosphere, the developers at Remedy refrained from divulging too much about the expansion's plot. We do know the expansion will take place after the events of the main game, when Jesse gets a call from the board alerting her of a disturbance that's causing the Oldest House's subterranean level, the Foundation, to go haywire. The bureau's Head of Operations Helen Marshall will also figure prominently into the expansion after she mysteriously went off on her own during the main storyline.
Control game director Mikael Kasurinen described the expansion as "an opportunity for [Remedy] to take the open-ended nature of the environments even further. The new area that you get to access is filled with mystery, discovery and player-driven exploration. As is the way of Control, it is embedded with symbolism. Exploring the foundation of the Oldest House is also about exploring the very foundation of Jesse herself. Who is she as the director and where does she need to from here?... Jesse will be tested unlike ever before."
Aside from the new storyline, there is a brand new skill in the expansion called "Shape" which is split into two forms: "Create" and "Fracture." This new ability is integral to the expansion not only because it's a new combat ability, but you'll need to use it to solve puzzles called "Rituals."