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)

In an underground world where tunnels extend everywhere, even though they live in dark and confined spaces, people wear protective clothes and lead quiet and enjoyable lives. Patema, a princess in her underground village, loves to explore the tunnels. Her favorite place is a "danger zone" that her village prohibits people from entering. Even though she's scolded, Patema's curiosity can't be held back. No one ever explained what the supposed danger was. On her usual trip to the "danger zone," Patema faces unexpected events. When hidden secrets come to light, the story begins to unfold. 
(Source: Anime News Network)

An OVA based on the Hoshi no Umi arc of the manga spanning chapters 26-34. 
 
This arc focuses on Rin Azuma, a Jiang-Shi yokai resident of Sakurashin adjusting to her new life in the city and working for a ramen shop. One day her former friend Zakuro Kurumaki invades Sakurashin to capture Rin and convince her to join her in destroying the city with the other yokai hunters as well as Enjin Hiizumi, the main antagonist of the series. Akina Hiizumi and the other members of the Hiizumi Life Counseling Office take action to save Rin, 'tune' the yokai controlling Zakuro and protect the city.
 
(Source: Anime News Network)

Purple Cow Studio Japan