While digging one of the many tunnels for the Moscow metro, Soviet workers unearth ruins of a dungeon. The relic is the key to the secret location of the priceless ancient library that belonged to Ivan the Terrible. Decades later, the foreman’s grandson Ilya discovers the ancient relic in a pile of old junk. What Ilya doesn’t know is that the search for the lost library has been going on for centuries, and now very powerful people are after him. Oblivious to the true value of the family heirloom, he soon learns about it from a mysterious stranger. Ilya and the mysterious stranger decide to try their luck in finding the library. They are soon joined by a beautiful philologist Arina, who takes it upon herself to decipher the encrypted message etched upon the ancient artefact. In search for clues that bring them closer to the coveted discovery, the trio goes on a quest that takes them to dangerous and obscure locations above ground and under water all across Russia, even to the Kremlin’s secret catacombs.