In the film Attack of the Clones, it is revealed that Count Dooku was once a Jedi who served as an apprentice of Yoda. He was the master of Qui-Gon Jinn, Obi-Wan Kenobi's old teacher. On planet Geonosis, he captured Obi-Wan Kenobi, Anakin Skywalker, and Padmé Amidala and sentenced them to death. They were to be executed in a Geonosian fashion, but the executions were nullified by the sudden arrival of Jedi reinforcements, led by Mace Windu.
In the video game The Clone Wars, Count Dooku led the army of the Confederacy of Independent Systems, or the Separatist Army. He found the Force Harvester, an ancient Sith artifact, on Raxus Prime. He later found the pieces scattered by the ancient Sith Jedi and revived an ancient Sith weapon known as the Dark Reaper. The Dark Reaper was first defeated by a Jedi Knight named Ulic Qel Droma 4,000 years during the Great Sith War before The Phantom Menace. At the Sith Temple on Thule, it was re-defeated by Anakin Skywalker at the final confrontation of the video game.
The film explains that Count Dooku left the Jedi Order after the Battle of Naboo and went to the Dark Side where he adopted the Sith name "Darth Tyranus".
The final big battle scene in Attack of the Clones where the Confederacy of Independent Systems battle the Jedi and clone armies of the Republic has a memorable fight scene between Count Dooku and Yoda. Because Yoda's actions in the fight scene would have been impossible to do with the puppetry used to portray the character in the original trilogy, he was instead animated using computer-generated imagery.