The Cheetahmen is one of the many franchises in LEGO Dimensions. It is the subfranchise of Action 52.
A boy called the "Action Gamemaster" is depicted in the first Cheetahmen game's intro sequence, which serves as a framing device. The boy is shown playing a video game when he is abruptly pulled into his television by a robotic arm and meets the Cheetahmen. He is never referred to again during the game or in subsequent Cheetahmen titles, although the manual implies that he transforms into the characters, one after another.
A backstory is provided in a comic book that was included with the Action 52 NES cartridge. A mad scientist named Dr. Morbis kills a mother cheetah while on a safari in Africa and takes its three cubs to his laboratory. Subjected to genetic experimentation, the cubs transform into human-cheetah hybrids who eventually turn on Dr. Morbis after gaining awareness of his true nature and future plans. Dr. Morbis responds by creating an army of various human-animal hybrids (known as "Sub-Humans") to counter the threat posed by the self-aware Cheetahmen. Dr. Morbis does not appear during gameplay, though other villains from the comic book do appear. The Genesis game provides a slightly different premise where the Cheetamen must rescue cheetah cubs from Dr. Morbis.