The adventures of Leo, a young lion cub who becomes king of the jungle when his father, the previous king of the jungle is killed by a human hunter.

An American in Paris lives by sponging off his working friends, and throws a party using borrowed money when his rich American aunt dies, believing firmly in his horoscope.

Leo is the lion king of the jungle, but unfortunately he doesn't know well what a king's job is. He believes it is his right to do as he pleases, even if it means disrupting the peace of others or being plain rude to them, as a king, Leo thinks everyone should look out for his well-being and comfort. One day, Leo meets a lion cub named Tooey whose mother has been recently taken away and who comes to Leo for help, being as selfish as he is, Leo turns the young lion away. However, after several different adventures, Leo learns what his place as a king is and how he must behave and treat the creatures who have almost lost all faith in him. Leo rallies the jungle animals to the rescue, and in the epilogue, the narrator Michael Donovan says that Leo asked Tooey's mother to be his queen, and the cub himself grew into a strong young lion, now called Leo II and one day he would be the Jungle King.

The story of a white lion cub growing to become the king of the jungle. The movie is an edited version of the 1965 Japanese television series of the same name based on the eponymous 1950s manga. The first 2 minutes of this film are taken from the Jungle Emperor Leo (1965) TV series, while the rest is all original. Tezuka was very pleased that this film conveyed his story accurately, something he wasn't able to do with the TV series.