As the world is threatened by the sudden appearance of a mysterious alien mothership, best friends Koyama Kadode and Nakagawa “Ontan” Oran carry on about their high school life. But as they grow up, they face existential questions, learning adulthood’s complexities and that the true threat may not be from above. 
 
(Source: Crunchyroll) 
Note: Includes Episode 0, which chronologically occurs between episodes 16 and 17.

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