Based on the novel series by Andrzej Sapkowski, The Witcher follows Geralt of Rivia, an amnesiac witcher (problem solver and slayer of monsters) whose home, the Kaer Morhen citadel, is besieged by an organization called the Salamandra, who steal their valuable potions and get away. Throughout the game, Geralt must resolve people's moral dilemmas, connect the dots of his past and track down the Salamandra.

The game begins when Geralt of Rivia, one of a few remaining "witchers" – traveling monster hunters for hire, gifted with unnatural powers, is transported to the witcher stronghold of Kaer Morhen by fellow witchers who had discovered him unconscious in a field. Geralt remembers almost nothing of his life before that point. He is taken to Kaer Morhen, the base of the witchers, where he meets a sorceress named Triss Merigold. The castle is attacked by a gang of bandits named Salamandra, led by a criminal known as the Professor, a mage named Savolla who controls a large, praying mantis-like monster, and another mage named Azar Javed. The witchers and the sorceress manage to slay the monster and kill Savolla, but the Professor and Azar manage to escape with the mutagens that genetically alter the witchers. After curing Triss of the wounds she received while fighting Javed, Geralt and the rest of the witchers head off in different directions in order to find information on Salamandra.