(or "Works In Theory")
The five movies that form Universal's original "Mummy" series--hang on. Let's break this down properly. There are actually three "Mummy" series from Universal, with similar but not contiguous continuity. First, there's the original film starring Boris Karloff, in which the reanimated Imhotep uses Egyptian magic to seek out his reincarnated lover, Ankh-es-en-amon, and to punish those who stand in his way. Then, there's a collection of four films ("The Mummy's Hand", "Tomb", "Ghost", and "Curse", respectively) which follow Kharis, who shares an identical backstory to Imhotep but was reanimated not by magic, but by a drug brewed by the priests of Karnak (or Arkam in the later films.) Kharis guards the tomb of Ankara, who is essentially Ankh-es-en-amon but who doesn't get reincarnated until about halfway through the series. He uses no Egyptian magic, simply physical strength and invulnerability to kill those who would defile the tombs of Egypt. Finally, there's the 1999-2008 version, which uses Imhotep and his love for Anck-su-namun, but gives him terrifying supernatural powers and turns the secret order of priests from the second franchise into his jailers, rather than his masters.
Got all that?
So, now we can talk about this properly. The first "Mummy" series that actually has a storytelling engine is the second franchise. (The Karloff film, while excellent, is entirely self-contained.) In the tradition established by Universal's other franchises, unfortunately, the studio spent less time and effort on them as the series went on. Monster movie fans, it was felt, would attend solely on the strength of the title and Lon Chaney's presence as Kharis--why work hard on a script, pay high-end actors, and establish good production values? With the exception of "Hand", these films are exercises in plodding boredom, barely even livened up by the frequent murders the slow-moving mummy commits, and the stories are lazy and disinterested.
But the storytelling engine is quite different. Egypt already has numerous tropes and a certain mystique that makes it a natural setting for a series. Then the idea of a secret order of priests that has infiltrated every level of Egyptian society, consumed with the idea of getting vengeance on a Western society that defiled their tombs and made off with their treasures--and an order that can re-animate the dead, no less? That's a rock-solid basis for at least one movie all on its own. The idea that their champion is kept alive as much by love for his long-dead Ankara as by the sacred tana leaves, and that he longs to drink enough of the heady brew to cast off the shackles of the priests' control and decide his own destiny? That's a tension that can build off of the priests of Arkam and come to a head in later stories. Then, adding to that all, we have Ankara herself, who's in a new body with a new life and who may or may not be willing to go back to the half-decomposed Kharis, and as the heroes--a two-fisted archaeologist, his street-wise Brooklyn buddy, their stage-magician business partner "The Great Solvini", and Solvini's daughter, a spunky trick-shot expert with a short fuse and a crush on the archaeologist. (The biggest mistake the series made was in getting rid of these characters in the last three movies.)
This is the engine behind the franchise, and it's a potentially great one. Arguably, Kharis would be a more compelling enemy if he could talk, and he might want to be a bit less, um...shambly...if he wants to be menacing, but those are minor changes. The point is, just because bad stories were told using this storytelling engine doesn't mean it's a bad engine.
The 1999 remake borrows liberally from all five previous films (with expert skill--the remake is almost a distillation of every good idea in the preceding movies)...but what's odd about it is that its sequels ("The Mummy Returns", "Tomb of the Dragon Emperor", two Scorpion King flicks, and a short-lived animated series) confuse the series' mythos with its storytelling engine. All of the later installments focused on Imhotep, Anck-su-namun, their ancient adversaries, the order of priests, other mummies that might happen to be in the vicinity...while the actual storytelling engine is simply, "two-fisted treasure hunter and spunky researcher seek out supernatural evils alongside her ne'er-do-well brother, all done in an 'action-comedy' tone." In the final iteration of the "Mummy" storytelling engine, the actual mummy, while brilliantly done, isn't necessary to the storytelling engine at all. Sometimes what you get on the screen isn't actually a representation of the potential of the series.