I have been reading many ppl saying that Dooku was already in league with Sidious during the evenets of TPM. That is rubbish. He joined the Sith AFTER TPM because Qui-Gon dying was the final straw. Also, how could he have been a Sith when there were already two, Sidious and Maul? Sidious needed a new apprentice after Maul died and didnt exactly have time to train someone from birth so he did the next best thing, converted an experienced Jedi Master who wanted out of the order and had been tinkering with Sith teachings for most of his life.