While studying the book of Omens, Lion-O is suddenly transported
back in time. He witnesses key
historical events through the eyes of his ancestor Leo, whose body he inhabits. In the distant past, the Thundercats served
as Mumm-Ra’s army, and Leo as his second in command. Leo intends to overthrow his master and
liberate his people. To do so, he plans
to intercept the coveted “warstone”, an item that is to be delivered to Mumm-Ra
upon its recovery. Once the stone is in
his possession, Leo will confront Mumm-Ra and inspire the other species under
his rule to rise up against him. Thus
begins the Thundercats existence on Third Earth.