Like many similar 1980s animated series, ThunderCats had a simple and predictable structure. It offered a long-running battle between good (the ThunderCats) and evil (the Mutants) set in a milieu that combined high technology and hand-to-hand combat. The ThunderCats are humanoids with distinct feline appearances while the Mutants are a rag-tag of uglies.
The action is set on a planet called Third Earth, a wild and violent world (which is our own planet earth) . Both the ThunderCats and the Mutants are non-native, the ThunderCats fled there when their home planet, Thundera, became unstable and the Mutants came from Plundarr. While the entire population of Thundera fled the disaster, the ThunderCats nobles number only seven (eight including Jaga, who died enroute). They dwell in a substantial fortress built in the shape of a cat, the Cat's Lair.
The leader of the ThunderCats is Lion-O. During the trip to Third Earth, his suspended animation pod malfunctioned, while only a youth on Thundera he arrives on Third Earth physically an adult while mentally still a child. Despite this problem Lion-O carries the group's super weapon, the Sword of Omens. When unused the sword was no larger than a dagger but extends to full size in battle. Embedded in the hilt of the Sword of Omens is the Eye of Thundera, the source of the ThunderCats' power. With the power of the Eye, Lion-O could call upon a variety of powers and abilities from the sword, including "Sight Beyond Sight" (the ability to peer at faraway goings-on and pierce the veil of the dimensional boundaries) and the "Cat's Growl" (a warning when danger was near and/or other ThunderCats were in trouble). With a cry of "Thunder! Thunder! Thunder! THUNDERCATS! HO!", Lion-O could summon the other ThunderCats to assist him in times of great danger.
The group's original leader, Jaga, did not survive the trip, piloting the ship towards Third Earth whilst the other ThunderCat nobles went into suspended animation. He appears in the form of a spiritual apparition, providing Lion-O with counsel when needed.
Second to Lion-O is Tygra, an older male and the brains of the group, acting as a foil to Lion-O's intemperate nature. He is armed with a whip which with a special twist can render him invisible, as well as other super-natural abilities but rarely demonstrates them.
After Tygra is Panthro, the strongest of the group and also the most technically adept. Easily the best fighter of the group, Panthro fights with nunchaku and is the designer/mechanic/driver of the group's oversized vehicle - the Thunder Tank.
Cheetara is the only adult female (for the first series). She can run extremely fast, fights and vaults with a quarterstaff, and, by running fast in a small circle, she can see into the past, though it is extremely taxing upon her.
There are two youngsters, Wiley-Kit and Wiley-Kat, who are constantly causing problems, getting into trouble and needing rescuing. They are armed with a "utility belt" of sorts, filled with various capsules and globes that perform a wide variety of tricks. They also wield lariats for ensnaring and the like.
The final member of the group is the comedy character, a curious hybrid of species. He is small, fat and named Snarf. Snarf was Lion-O's nursemaid, but when Lion-O was aged to maturity, Lion-O didn't feel he needed a babysitter anymore. Still, Snarf is the most loyal of ThunderCats (yes...he is a ThunderCat) and their most underrated member.
As well as the original ThunderCats the second series added Bengali (the white tiger blacksmith), Pumyra, Lynx-O (the blind ThunderCat). Other allies include Mandora, the Evil-Chaser (an intergalactic police officer), Hachiman (a samurai swordsman), and Snarfer, Snarf's nephew and technological genius.
Opposing the ThunderCats are the Mutants, living in Castle Plundarr. Their lineage is more diverse than the ThunderCats and includes a number of less well regarded species. In common with most cartoon 'bad guys' they are extraordinarily incompetent, led by the silibant Slythe. They include Monkian, Jackalman and Vultureman. In order to offer a significant challenge to the ThunderCats they are led by Mumm-Ra. He usually resides in a pyramid as a withered corpse-like being but by reciting a piece of doggrel he can be 'pumped-up' into a much more muscular blue-skinned being to offer the ThunderCats an episode long challenge before being driven back. Sometimes other evil entities arrive on Third Earth, before being forced back into space. Mumm-Ra also has a dog, blue and bulldogish named Ma-Mutt.