From the start, “Adventure Time” was at war with its own potential. Creator Pendleton Ward transformed the format of an animated children’s show into a poetic rumination on childhood, merging geeky sensibilities with sophisticated imagery and a freewheeling mythology with no real parallel in popular culture. It was a transformative approach to all-ages entertainment, but it was also too good to last.

In between its zany punchlines, the show had profound things to say about Cartoon Network’s core audience of drooling toddlers and hyperactive pre-teens — as well as the adult nostalgia for those halcyon days. But none of that fits in a marketing box, and as ratings dwindled, Ward left the show after Season 5. Showrunner Adam Muto kept the DNA intact, but “Adventure Time” faded into oblivion by the time it was officially canceled in early 2017. Since then, episodes have been released in short bursts, with tiny narrative...