With the arrival of Lion (Tasuku Sato), the peaceful, routine life of the Komori household quickly begins to unravel.

Hiroto (Yuya Yagira) wakes up to the screams of his younger brother Michito (Ryota Bando), who has autism spectrum disorder. When he rushes downstairs, he finds their living room in disarray, with Lion causing trouble and Michito panicking. After struggling to calm the two down all morning, Hiroto starts to worry about keeping Lion in their home.

Later, a second message arrives on Lion’s phone from someone who appears to be his mother:
“At the square with the soft-serve ice cream, when the bell rings.”

Unable to decipher the message, Hiroto decides to take Michito and Lion out for the day. Going to the library, bakery, and park is part of the Saturday routine for Hiroto and Michito. When they arrive at the library, Hiroto happens upon a picture book and realizes it hints at the place mentioned in the mysterious message. He begins to think that by going there, he might be able to meet Lion’s mother. Urging Michito to stay with Lion, Hiroto hurries to the location specified in the message.

But just as he arrives at the designated spot, Hiroto receives a call from the police.