After a drunken night on the town, an ordinary Seoul businessman, OH Dae-su, wakes up in the morning to discover he has been locked up without explanation in a mysterious prison. After fifteen years, Dae-su suddenly and inexplicably finds himself freed. A voice on a cellphone taunts him into discovering who kept him imprisoned. Dae-su plunges into a world of competing vengeance, his own drive for revenge matched by the equally implacable and Byzantine...

A meek scientist meets and eventually learns to deal with his double, who, though physically identical to him, is frighteningly opposite in nature.