Environmental activist Natalia Mirova investigates mysterious methane releases in Lake Baikal, Russia, despite opposition from her colleagues. An old fisherman named Leonid brings her a water-damaged book listing people who have disappeared in the lake over centuries. During a dive with her team, they discover a submerged village and an ornate metal box. When her colleague Dmitri touches it, he vanishes mysteriously. Seismic activity around the lake intensifies as Natalia deciphers the damaged book, revealing an ancient covenant: every seven years, a chosen person must willingly go into the lake to prevent catastrophic earthquakes. The cycle was disrupted when Soviet authorities flooded the original village of Enkheluk. Returning alone to the underwater shrine, Natalia meets the Keeper who reveals she comes from a lineage of "messengers"—including her mother who hadn't drowned accidentally. Faced with imminent geological disaster, Natalia accepts her role but does so as a scientist, ensuring her equipment records data that might help end the cycle.