Lion-O, always outdone by his adoptive older brother Tygra, undergoes the ritual to be chosen by the Sword of Omens as the next king of Thundera. His father, Claudus, does not feel his son is ready because he is too lenient and easily distracted by flights of fancy like the myth of ancient technology. Soon after, King Claudus welcomes the return of his friend Grune and holds a celebration in both his honor and in memory of the fallen Panthro. During the event, Lion-O defends a pair of captured Lizards and convinces his father to release them.

An unforeseen betrayal by a former comrade brings destruction upon the Cat Kingdom. While King Claudus leads the fight against General Slithe and his lizard army, the evil sorcerer Mumm-Ra attempts to seize the Sword of Omens.

The ThunderCats accompany Panthro to Cloud Peak Mine to search for Thundrilium for the Thundertank. Along the way, Panthro and Grune's past is explored, from their first encounter to the reason behind Grune's betrayal.

While traveling through snowy mountains in search of safe passage, Tygra and Lion-O come upon a lost tiger village and in it they find Tygra's real father. Tygra learns why his father sent him away as a baby — to save his life. But all is not as it seems in this village as the brothers learn these tigers are cursed and Tygra may be the answer to saving them — but at the cost of his own life.