To un-invert, the Protagonist travels back in time to the freeport in Oslo, fights his past self, and enters the turnstile, followed by Neil and a healing Kat. In Oslo, Priya tells him Sator now has all nine pieces of the "Algorithm", a device that future antagonists need to invert the entropy of the world and destroy its past. She planned for Sator to get the artifact to reveal the other eight pieces in preparing his dead drop. Recalling an earlier conversation with Sir Michael Crosby, the Protagonist realizes it is a nuclear hypocenter detonated on "the 14th" in Sator's hometown of Stalsk-12.