Imagine, that the roles of whales and humans were swapped. So whales (or whale-like creatures) would be rather small, able to talk and smart. Humans would be large, stupid and hunted by whale-people. Then shake this all in the slightly insane head of Shintaro Kago and you get this peculiar one-shot, making you see how our actions affect nature.
The series 'A Certain Hero's Death' has been categorized as 'mature', therefore may contain intense violence, blood/gore, sexual content and/or strong language that may not be appropriate for underage viewers.