Omens, Part 1: Lion-O undergoes the ritual of being chosen by the Sword of Omens to be the next king of Thundera - although his father Claudus does not deem him ready, his older brother Tygra outdoes him in every way and Thundera's people dislike him due to his beliefs in technology as well as peace between the species.
Omens, Part 2: Thundera is attacked by the Lizards who've acquired technology (in the form of Humongous Mechas), joined by (former) Thunderian general and Drunk with Power traitor Grune and ancient evil Mumm-Ra. Mumm-Ra tries to obtain the Sword of Omens, but Lion-O, Tygra and Cheetara manage to take it, and escape thanks to a Heroic Sacrifice from Head Cleric Jaga, who sends Lion-O out to search for the Book of Omens.
Ramlak Rising: The Thundercats set out on a journey to go to Mumm-Ra for Revenge, but are trapped in the Sand Sea on the ship of Captain Koinelius Tunar, who seeks to destroy the sea creature Ramlak.
Song of the Petalars: While hiding in a thornbush forest, the Thundercats meet the Petalars, a plant race who seek the Valley of Winds.
Old Friends: In order to recharge the Thundertank, the Thundercats need to obtain Thundrillium from a mine controlled by the Lizards and The Driller. Panthro's history with Grune is revealed.
Journey to the Tower of Omens: The Thundercats find the Tower of Omens, wherein the Book of Omens lies, and the Thundercats fight Mumm-Ra to obtain it.
Legacy: Lion-O's soul is sent into the Book of Omens, and experiences the history of his ancestor Leo, serving Mumm-Ra alongside the cats on his alien spaceship, with all other species held captive. Leo leads a rebellion with the other species and his girlfriend Panthera, and overthrows Mumm-Ra as the spaceship crashes into Third Earth.
The Duelist and the Drifter: Lion-O enters a town, looking for supplies to repair the Thundertank. He gets challenged by The Duelist, a master swordsman who takes the swords of everyone he defeats. Lion-O also meets the Drifter, an eccentric rabbit man who repeatedly offers him advice while claiming not to care what happens to him.
Berbils: The Thundercats meet a robotic bear species called the Berbils, who help the Thundercats, but are assaulted by a slave trader named Conquedor.
Sight Beyond Sight: The Thundercats arrive at the temple of the Elephants, where one of the three magical stones is located. However, Lion-O can't find it using the Sword of Omens' power of Sight beyond Sight.
The Forest of Magi Oar: The Thundercats are sent to the titular forest, where they meet the Wood Forgers, who have a magic school and paper mill in the forest. However, they are in conflict with giant bird Viragor, who seeks to protect the forest from the Wood Forgers.
Into the Astral Plane: The Thundercats return to the Elephant temple, now aware that they need to open up a gate to the Astral Plane in order to get the stone. Cheetara's backstory of joining the Clerics is revealed.
Between Brothers: Inside the Astral Plane, tension rises between Tygra and Lion-O, but they eventually unite to fight against Mumm-Ra in the Astral Plane. Meanwhile, the rest of the Thundercats and the Elephants defeat Grune and the Lizards.
New Alliances: As Lion-O frees more Lizard soldiers from their tyranny, Mumm-Ra and Slithe recruit Kaynar and Addicus as new generals for their army.
Trials of Lion-O, Part 1: The Thundercats are ambushed by the strengthened Lizard army and its generals. The Thundercats are captured, the sword of Omens is stolen and Lion-O seemingly dies, but the Spirit Stone grants him a second chance at life, provided he can pass four trials...
Trials of Lion-O, Part 2: The surviving Thundercats infiltrate Mumm-Ra's headquarters, attempting to retrieve the Sword of Omens. Lion-O returns from the dead to save them and successfully retrieves the Sword of Omens and the Thundercats escape.
Native Son: Lion-O and Tygra find the lost Tiger Clan high up in the mountains. Tygra is pleased to see his father Javan and species, but Javan tries to make them leave because of the Tiger Clan's curse...
Survival of the Fittest: Panthro and Cheetara try to teach Wilykat and Wilykit respectively to hunt, while the backstory of the Thunderkittens is revealed.
The Pit: The Thundercats reach a city inhabited by dogs, who have a cat, Pumyra, captured as gladiator, and Lion-O tries to free her. Meanwhile, Wilykit and Wilykat run into Tookit, who steals their flute and flink.
Curse of Ratilla: The Thundercats arrive at Mount Plun-Darr, where Thunderian cats work in the mines as slaves to excavate the Sword of Plun-Darr for Ratar-O and the Rats.
Birth of the Blades: The Thundercats defeat Ratar-O, as Mumm-Ra arrives on the scene to try to take the Sword of Plun-Darr. The story behind the Sword of Plun-Darr and the Sword of Omens is revealed.
The Forever Bag: The Thunderkittens continue to look for Tookit, who reveals he has the Forever Bag, and manipulates them (and other Street Urchins) to assist with his thievery. Eventually, the children trick Tookit into getting arrested by the Constable, and take off with the Forever Bag.
Recipe for Disaster: The Thundercats meet Ponzi, whose "miracle elixir" turns out to be the only weapon capable of harming Mumm-Ra, who attacks them in the form of a Nigh Invulnerable dinosaur.
The Soul Sever: The Thundercats run into The Soul Sever, who seeks to bind the souls of his family to robotic bodies using the Book of Omens.
What Lies Above, Part 1: The Thundercats go up in the sky to the floating city Avista in order to get the Tech Stone from the Birds' leader Vultaire, who refuses to give them the stone.
What Lies Above, Part 2: Mumm-Ra, also going for the Tech Stone, attacks the Thundercats and Avista while also compelling Vultaire to join him and Pumyra revealing herself as The Mole.