The ThunderCats continue their quest for the Book of Omens, which leads them to the fabled Tower of Omens. However, they will first have to get past a series of deadly traps in order to reach it. Meanwhile, Mumm-Ra grows tired of waiting and forces Jaga to reveal the Book's location, and plans to go there himself to take it at all costs.

Lion-O ventures into the Book of Omens where he becomes the incarnation of his renowned ancestor, the valiant Leo, who led the rebellion against Mumm-Ra in a previous millennium.