Leo the Lion is a sequel to the Japanese-American co-produced series "Jungle Emperor", or Kimba the White Lion. Osamu Tezuka had always wanted his story of Kimba to follow Kimba's entire life, and the Jungle Emperor/Kimba series was such a hit in Japan that Dr. Tezuka produced a sequel, without his American partners, in 1966.
Making the series without a co-producer gave him complete creative control. For example, Dr. Tezuka changed the conclusion of his original manga story to a happy ending.
Leo the Lion does not follow immediately from the end of the Kimba series. Instead, the story begins a couple of years following the end of the previous series. To English-speaking audiences, the behavior of the title character is inexplicably out of line with what was established in the first series. At the end of the first series, in the original Japanese script, Kimba promises to keep his animals separate from humans. It is this promise that drives the seemingly hermit-like Leo in this series.
As the series unfolds, the focus shifts from the title character to one of his cubs, the male named Rune. This series as a whole is about Rune's growth, from a whining weakling to a confident leader.