| Director | John Erick Dowdle |
|---|---|
| Genre | found footage, horror film, thriller film |
| Runtime | 93 min |
| Country | United States |
| Budget | $10 million |
| Box office | $41.8 million |
Polymath and archaeologist Scarlett Marlowe continues her late father's search for the legendary philosopher's stone, an alchemical substance discovered by Nicolas Flamel allegedly capable of turning base metals into gold or silver and granting eternal life. After obtaining the "Rose Key" artifact in an Iranian cave, she has a vision of a hanged man before escaping.
Scarlett travels to Paris, where her search becomes the subject of a documentary filmed by Benji. Assisted by her friend, Aramaic translator George, she uses the key to solve a riddle that points them towards the Catacombs of Paris. When they are unable to access a specific tunnel, a stranger directs them to a nearby club to find cataphile Papillon. The trio visit the club, recruiting Papillon, his girlfriend Souxie, and their friend Zed to help in their excursion.
Papillon leads the group to an off-limits entrance. Scarlett insists on taking a shortcut through a side tunnel, causing Papillon to refuse, warning her that people disappear inside, including his own friend, La Taupe. After taking an alternative route that collapses behind them, they find themselves facing the same tunnel and are forced to go through it. Inside, they find La Taupe, who tells them that going further down is the only way out. They eventually find a tomb holding the "philosopher's stone"; Scarlett seizes it, fulfilling her search. Papillon's group inadvertently triggers a booby trap that causes the ceiling to collapse and seemingly separates La Taupe.
Scarlett uses the stone to heal Souxie's wound. They find a drawing of a door on the ceiling along with a Gnostic Star of David, symbolizing "As above, so below", which reveals a hidden opening in the floor. Going through, they find a tunnel marked with the phrase "Abandon all hope, ye who enter here".
They enter a inverted mockery of the catacombs, where La Taupe murders Souxie before disappearing. As they descend deeper, Benji and Papillon are murdered by spirits representing their past sins. The remaining three continue to flee, when a demon bites George's throat. Unable to heal George using the stone, now revealed to be false, Scarlett returns it and discovers that its power resides within her. While returning to George and Zed, she sees the hanged man and recognizes him as her father's vengeful spirit; she apologizes for neglecting to have a final conversation via telephone with him and he vanishes. She returns to George and heals him with a kiss.
The trio is chased to a dark hole, where Scarlett tells them that they must confess their sins and jump in. George admits that he failed to save his brother from drowning and Zed confesses that he has a child he refuses to claim. They jump and find a manhole at the bottom; opening it, they invertedly emerge near Notre-Dame. Scarlett and George embrace while Zed walks away, finally safe.
In an ending log, Scarlett states her search was to never seek or obtain any material treasure, but only the truth.