After the war, concerned brotherHide promises his critically ill sisterMiyoko that he will arrange for her to see a real elephant. But when he goes to the zoo to find one, an old man tells him of the tragic day when the zoo animals were slaughtered.

Source: The Anime Encyclopedia

Note: the story is based on true historical events that took place in the Ueno Zoo in Tokyo, where dangerous animals were slaughtered during WW2 in order to prevent them from wreaking havoc in case Allied bombings would let the animals escape.