This is an online distribution of the prologue of the movie, illustrating the first day of the entire story. 
A world, forever beyond your expectations. 
In a dark, cramped, underground world of endless tunnels and shafts, people wear protective suits and live out their modest yet happy lives. The princess of the underground community, Patema, goes out exploring as always, inspired by her curiosity of the unknown depths of the world. 
Her favorite spot is the "danger zone", an area forbidden by the "rule" of the community. Despite being frequently chastised by her caretaker Jii, she cannot hold back her curiosity for the reason behind the rule, because no one would tell her what the "danger" was. When she approaches the hidden "secret", the story begins. 
(Source: translation of a synopsis from the nicovideo news)