We start of on the planet of Dathomir where we see Darth Sidious kidnapping Maul the child of Mother Talzin the leader of the night sisters a group of female witches.
Mother Talzin was furious when she found out her son was taken from her so she stopped Sidious from being able to learn more of there mystic arts.
Maul was taken to a training facility on Mustafar the lava plant wher he would be taken care of by the droids for the first years of his life to be prepared to be trained by Darth Sidious.