The continuity in the Masters of the Universe franchise ensures that the stories told about the brand's characters and toys fit together into a consistent narrative. However, as the franchise has grown and evolved since the early 1980s, storyline changes have been made resulting in several variations of that continuity.
Prior to Filmation's He-Man and the Masters of the Universe animated series in 1983, the franchise's backstory was largely defined by Mattel itself in the minicomics packaged with the toys. This initial continuity is commonly referred to as "pre-Filmation" or "Mineternia" (a portmanteau of "minicomics" and "Eternia") to distinguish it from the continuities that followed.
In the early development of the original Masters of the Universe line, Western Publishing, then owned by Mattel, contracted Donald F. Glut to write the backstory for the toys, which would be packaged as four minicomics with the action figures. In Glut's story, He-Man is simply a powerful savage who lives in the jungles of Eternia until he ventures out to protect Castle Grayskull from the evil forces seeking its power; Skeletor is an extradimensional demon trapped on Eternia during the Great Wars, seeking to help his race join him and conquer the realm. For his narrative Glut created both the blonde "warrior-goddess" Tee-La and the green-skinned, cobra-themed Sorceress
Following the original four minicomics, DC Comics obtained the license for Masters of the Universe comics. The second wave of minicomics largely followed the premises introduced by Glut's storyline, with He-Man as a heroic, axe-wielding barbarian roaming around fighting evil and answering the summons of the Sorceress, now known as the "Goddess." However, DC's full-size Masters of the Universe comics revise several of Glut's premises. He-Man is now the alter ego Prince Adam, the son of Eternia's king and an astronaut from Earth. Although Adam and Cringer transform into He-Man and Battle Cat by entering the Cavern of Power, and the Sword of Power is still an artifact rarely in He-Man's possession, these stories lay the foundation for the first animated series.
The Masters of the Universe backstory was fleshed out and refined in December 1982, when Michael Halperin was asked to develop a show bible for He-Man and the Masters of the Universe. Although the bible presents many story elements that did not make it to television, it critically introduces the concept that the Sword of Power is entrusted to Prince Adam, who uses it to become He-Man by the power of Grayskull. Other ideas, such as Orko and the relationship between the Sorceress and Zoar, make their first appearance in the bible. With the debut of the cartoon, Masters of the Universe stories in other media gradually shifted to resemble the cartoon's continuity. The cobra-themed Goddess gave way to the falcon-themed Sorceress, and He-Man became less of a noble savage and more of a superheroic nobleman.
The Filmation continuity was slightly revised to accommodate the development of the Evil Horde toys and the Princess of Power spinoff line. It was suddenly revealed that Prince Adam was one of a pair of twin babies born to King Randor and Marlena; that Adam's sister Adora was taken by Hordak to Etheria; that Skeletor had been Hordak's pupil; and that Adora had always been intended to receive a Sword of Protection akin to the Power Sword, so that she could become She-Ra. In spite of this considerable retroactive continuity, the She-Ra: Princess of Power (cartoon) show co-existed in a single continuity with its predecessor, with He-Man characters frequently crossing guest-starring. This did, however, create a rift with the continuity of the toy lines and their minicomics; Filmation freely disregarded these sources whenever it was more convenient to, for example, place one of Skeletor's warriors on Etheria as a member of Hordak's forces.
Following the end of the He-Man cartoon, Masters of the Universe products generally followed the backstory of that show except to avoid crossing over with the Princess of Power toy line. In the minicomics, for instance, Hordak would cross over from Etheria to battle He-Man, but She-Ra and Catra would not be involved. With the demise of the She-Ra series, the franchise's continuity lacked a central tentpole, and several variants of "what happens after the cartoons" emerged.
Both the 1989 He-Man franchise and the unproduced He-Ro Son of He-Man and the Masters of the Universe series were intended as continuations of Filmation continuity. The final Masters of the Universe minicomics began to explore Eternia's prehistory and the mystery of King Randor's brother Keldor. Whether any (or all) of these elements can be reconciled with Filmation continuity is purely subjective.
In 2002 Mattel relaunched the Masters of the Universe brand with a new toy line and various tie-in media, including comics and an animated series. The continuity of this era was much more internally consistent, with the toys and media following the same backstory. Although the 2002 continuity is strongly influenced and very faithful to the 1980s stories, many elements were redesigned or revised, resulting in a new (albeit familiar) history of Eternia. Whereas the late '80s minicomics clearly established that Keldor was Randor's long-lost brother but only hinted that he became Skeletor, this era unequivocally states that Skeletor was once Keldor without directly addressing his relationship to Randor. Certain properties, such as He-Man's ancestor King Grayskull, are introduced for the first time. Others, such as the Evil Horde, are barely used, due in part to the toy line's demise in 2004.
The Masters of the Universe Classics toy line seeks to present a new continuity by reconciling the best of all previous continuities, most notably the versions from the comics, the Filmation era, and the 2002 relaunch. It is established in this franchise, for the first time, that He-Ro and King Grayskull are contemporaries, that the Goddess and the Sorceress are two distinct individuals, and that Keldor is both the brother of Randor and the former identity of Skeletor.