Siblings, Atlas and Cherry Blossom, two unicorns who come from the port city of Poninawa in southern Equestria, are forced to flee when Zebra forces invade the city, setting it ablaze in a shower of Napalm and Spell-Fire.
With their mother deceased and the war becoming ever more heated, the sibling's do what they can on their long exodus to Ponyville. Tragedy strikes, the Megaspells fall, casting their radioactive cloud over the land as the war draws to its devastating climax. Atlas will do whatever he can to keep his sister alive.
Based on the book, Grave of the Fireflies by Akiyuki Nosaka.
And the Studio Ghibli film of the same name by director Isao Takahata.