The film's trailer would have you believe that As Above, So Below is faux documentary about a group of archaeologists who venture below the streets of Paris into the infamous catacombs - a massive underground labyrinth, now largely walled off, that was used as a burial site in the 18th century. While down there, they unwittingly step into some sort of a hell dimension that plays upon their worst fears and memories, bringing about psychological torture of the worst kind.
All of that is true, but most of it doesn't really come about until the last 20 minutes of the film. What the marketing left out, is actually one of the things that I found most interesting about the film's set-up. The story focuses on Scarlett Marlowe (Perdita Weeks), an archaeologist obsessed with completing the research of her late father. The research in question is the quest to finally find the famed Philosopher's Stone which was supposedly made by Nicholas Flamel back in the early 1400's. Her findings lead her to Paris, where she is joined by a cameraman (nobody really says why, which is a bit messy), intent on documenting her quest. She convinces a colleague, George (Ben Feldman) to accompany her, recruits a group of bohemian Parisians who have made a hobby out of renegade-exploring the tunnels to be her guides, and off we go.