Dr. Vitus Werdegast is one of Hungary's greatest psychiatrists, and has an intense fear of cats. A veteran of World War I, he was stationed at a fort towards the end of the war commanded by Hjalmar Poelzig. When Poelzig sold the fort to the Russians and snuck away, he left Werdegast and the others to die.
Werdegast however survived and was taken to a horrible prison camp near Lake Baikal. He spent the next fifteen years imprisoned there, in a place where he says "the soul is killed--slowly", waiting to take his revenge on Poelzig.
Once released, he sets out to find Poelzig and take his revenge. He learns in Budapest that Poelzig told Werdegast's wife--Karen--that Werdegast had been killed. Werdegast knows that Poelzig always wanted Karen from the first moment he saw her, and that he seduced her after he was captured.
He shares a train compartment with Peter and Joan Alison, and they are all forced to take shelter during a storm at Poelzig's home, built on the ruins of the same fort he'd betrayed. Werdegast confronts him and demands to know where his wife and daughter are.
Poelzig shows Werdegast his wife's body, preserved in a glass case (like so many others). He tells him his daughter died too. Werdegast wants revenge, but bides his time, aided by his servant Thamal. He notices Poelzig has taken an interest in Joan, and intends to fight for her freedom. To that end, they play a game of chess, both physically and metaphorically.
Werdegast manages to help Joan, but she tells him that his daughter is not only alive but Poelzig's wife. Werdegast searches for her, only to find her having been killed, devastating him. He fights and overcomes Poelzig, before skinning him. Later, when he's trying to help Joan, her husband thinks he's attacking her and shoots him. Joan tells Peter that Werdegast only wanted to help. The mortally wounded doctor tells the couple to flee, before throwing a switch that ignites dynamite, blowing up the house.