Retired police officer Ptakhin, delving into the private archives of an abandoned mine in the Irkutsk region, discovers in the diary of the late chief geologist traces of a mysterious story that began in pre-war times. Trying to figure out what is said in the encrypted part of the archive, Ptakhin contacts the family of the deceased and, together with his son, tries to unravel the secret. At the same time, a gang of criminals arrives in Irkutsk with a task: to find and deal with a certain former “cop”. From the photo, the leader identifies his longtime enemy - investigator Ptakhin. The gang follows on the heels of Ptakhin and his team, who decided to go down into the mine. But neither of them even suspects what they will have to face in the darkness of the dungeon!..