Summer

The events go back to when Saitama was roughly eighteen months into building his power. In Z-City, frightened residents pack into Shelter Nr. 7 to escape a Mysterious Being, a cicada larva said to have lived some 170,000 years. Two C-Class pros, Red Muffler (Rank 140) and Grad School Graduate (Rank 299), can barely land a hit on it, and the cocky rookie Darkness Blade fares no better. All three retreat inside to jeers from the crowd, with Grad School Graduate close to giving up. Red Muffler argues the thing simply cannot be beaten and proposes waiting out its natural lifespan, which someone points out could run 170,000 years.

Saitama, who tailed the heroes only to fight the monster, realizes he has been herded into the shelter. When he tries to leave, panicked bystanders assume he is suicidal and grab at him, one yanking out a fistful of hair. A Hero Association staffer keeps him inside, insisting over her megaphone that the flawless shelter is perfectly safe. Saitama then needs a restroom and learns the place has none, which he calls a serious flaw. An hour on, still holding it, he overhears that the beast has been reclassified Demon after downing two A-Class heroes, and that a subjugation team is three hours out. Fed up, he smashes through the wall to reach a convenience store and flattens the Cicada Larva the instant it crosses his path. In the present, he spots the mature form, now a full-grown adult of that same ancient species, drifting overhead and remarks that summer has come.

Red Muffler, Grad School Graduate, and Darkness Blade all fail to wound the Cicada Larva and fall back into the shelter. Trapped inside against his will, Saitama discovers the place has no toilet. Word arrives that the monster, now rated Demon, has beaten two A-Class heroes. Saitama breaks out and kills the larva in a single pass on his way to buy groceries. Years later, an adult cicada in the sky signals the season.

Summer (夏, Natsu) is a twenty-three-page bonus chapter tucked into Volume 3, and Viz keeps the same title. The exact release date is unlisted. The cast covers Saitama, the debuting-and-dying Cicada Larva, the rookie Darkness Blade, C-Class pair Red Muffler and Grad School Graduate, the Shelter Nr. 7 attendant, and the grown adult cicada. In sequence it lands between Brushing Up and A New Wind Blows.