Devilman: One Summer Day ("夏の日", literally "Summer Day" or "Summer Days") was a eighteen page Side story to the original Devilman, first included in the Deluxe Edition. It was one of three side stories to have been written. It took place early on on the original Devilman, taking place either shortly before or after his fight with Sirene, it showed the love triangle between the manga's three main characters Akira, Miki and Satan. A chapter of Devilman Grimoire has a similar plot to this one.
The story starts at a beach with Akira and Miki splashing about in the sea, whilst Ryo looked on at Miki with spite. Akira calls Ryo asking him to come in he says no and the two go diving, Ryo using his powers continues to watch them.
After swimming far out Akira and Miki surface and notice the darkened sky, suddenly a massive jet of water pulls Miki up and reveals itself to be a massive sea serpent demon named Doelemale, the demon starts to taunt Akira as it rips Miki's clothing away with his tongue, Akira transforms and the demon sprouts a pair of massive wings, dropping Miki into the water.
The scene suddenly changes to Ryo having fell asleep on the now empty beach, he has a dream (which is actually a premonition of the future) and see's Akira standing before him looking down on him with hate, before taking Miki's hand and walking into the afterlife.
Meanwhile Akira finishes the battle with the demon electrocuting it causing it to explode, he reverts to his human form and picks up Miki and replaces her swimming suit, she wakes and asks what was happening, Akira says that she passed out but was ok, afterwards the two embrace as the moon rises behind them.
- Akira Fudo/Devilman: A Devilman and the main protagonist of the Devilman series.
- Miki Makimura: A human girl who was Akira's love interest.
- Ryo Asuka/Satan: The leader of the demons, he fell in love with Akira and hated Miki for stealing his heart.
- Doelemale: A massive leviathan of a demon with a sadistic personality.