J.R.R. Tolkien's "The Lord of the Rings" trilogy is complex and large enough to be difficult to adapt. The animated attempt in the 1970's would be proof of that. Money, marketing, and multiple studios involved would lead to baffling confusion: "The Hobbit" was released by Rankin/Bass Productions, and "The Lord of the Rings" by United Artists and Ralph Bakshi a year later. A sequel to Bakshi's film was planned, but never funded: Rankin/Bass came to the rescue a couple years later, having always intended to make a sequel to "The Hobbit," and also excited to fill the void left by the Bakshi's theatrical hit. Not unsurprisingly, the result is a mess. Not as much as a mess as you would think, though. Despite the two earlier films being made by separate studios, the old-style of cel animation is so unusual today (written circa 2017) that a new viewer could squint and pretend they are in the same universe. General visual and audio design inspired by "The Hobbit" helps bring everything back to a vague sense of familiarity. In terms of story, "The Return of the King" does provide something of an ending to finally cap of the saga.Naturally, you are assumed to have watched both "The Hobbit" and "The Lord of the Rings" before attempting to watch "The Return of the King," and not doing so will leave you confused and bored. Even then, a large chunk of the story is left out: the movie switches between Gandalf and company preparing for a massive battle, and Frodo and Sam near the end of their journey to destroy the one ring that powers the dark forces. All told by an older hobbit reminiscing of the adventure to local children, already a storytelling strategy that doesn't bode well. Really, the story of Frodo and Sam (and Gollum, who guides them) overtakes the movie, making it feel as if there are only three characters. Of course, when they finally destory the ring, the massive battle becomes moot, and everything is over. It was an issue I had with the Peter Jackson films too, but at least there, the battles were big and cinematic and engaging. It's hard to carry a whole film with just Frodo and Sam remembering the Shire they grew up in, and arguing with each other about whether or not they ring should be destroyed. 
Indeed, not only is that drama more dull for young viewers, "The Return of the King" is also darker and more violent. Which makes its marketing, and reuse of whimsical music from "The Hobbit," all the more unusual and misleading. While animation is a slight improvement from "The Hobbit," I felt the visual design was a step down, despite borrowing heavily from the stylized art from that movie. "The Return of the King" is arguably the worst of the cheap but classic Rankin/Bass 2D animated fantasies. It's not completely a waste: if you were disappointed from the lack of an ending in 1978's "The Lord of the Rings," this provides something of value. But it feels like that's the only valid reason for the movie to exist, despite Rankin/Bass planning the sequel before Bakshi's film had released. It's really best left for completionists only.