"Lost Cat" (迷い猫, Mayoi Neko) is an extra chapter included with Volume 8.
In a cafe, Saitama is given a mission by a Hero Association staff member. Happy that he is finally being recognized, Saitama accepts the mission before even hearing what it is. The man asks him to find a cat, since he probably has no obligations at the time.
At the park, Genos is waiting for a Hero Association staff member. The staff member tells him of a mysterious being he kept as a pet, which ran away, and asks him to get rid of it. The staff member tells him that residents of the suspected area of where the monster would be will be evacuated under the pretense of a pollutants investigation, telling him the mission has to be done under absolute secrecy.
On the streets, Saitama looks for the lost cat, Torako. Saitama is given a drawn picture that tells him that Torako has white fur, a blue collar and is supposedly cute, but is still stumped. The young daughter of a Hero Association staff member is there to help identify Torako, but ends up yelling at him because he was complaining. She tells him that if he doesn't help, she'll get him fired. Saitama then tells her that he does not care whether or not he is fired. At the realization that she may have to do it alone, the girl starts crying, which causes Saitama to comfort her, saying he will help her after all.
Genos is walking down the evacuated streets, noticing a trail of blood, which leads him to an alley. In the alley, he finds two injured A-Class heroes, Peach Terry and Heavy Kong, who have received the same mission as him, but were defeated by the creature. Genos asks why they were not devoured and Peach Terry tells him that it was probably due to the fact that the creature sensed his arrival and fled. The creature, Demon Level - Grizzly Nyah, is then seen above Genos, clinging to the walls, ready to attack.
Saitama is still on the search for the lost cat, which he thought he found but was told otherwise. He then goes to an isolated area which is guarded by Tanktop Tiger and Tanktop Black Hole, who have no idea why they are guarding it. The two notice Saitama and become frightened, however they still tell Saitama he cannot pass and that only chosen heroes may go beyond that point. Saitama thinks that he is one of those heroes, due to the fact that he was requested for a job by a Hero Association staff member.
Saitama, still thinking that it was for his mission, is now in the isolated area. He comments about how weird rich people can be and notices a cat. Meanwhile, Genos bursts through a wall. Genos notices his master and thinks that he was chosen for the same mission as him. When Saitama asks Genos why he is there, he tells him that he is in mid battle and that his target is stronger than he anticipated. He then requested Saitama to leave it to him. Saitama then tells Genos to do his best and continues on with his search. Saitama asks the girl about the cat and tells her that a similar cat just came through.
Meanwhile, the A-Class hero Lightning Genji is seen fighting Grizzly Nyah. Lightning Genji manages to stun Grizzly Nyah while dodging it's attack. He then tells A-Class hero Stinger to attack, who then manages to sever it's right foreleg, and bring Lightning Genji back to his feet. A cracking sound is heard and the two heroes are surprised to find a Giant Grizzly Nyah behind them.
A great roar is heard and Genos notes that the Grizzly Nyah he had just beaten must have been the child, and that the mother must have given birth after it's escape.
Meanwhile, Lightning Genji and Stinger run from the "Mother Nyah" and notice a little girl running. The girl who was chasing her cat, Torako, is grabbed before she can get back her cat. Torako is then seen with a mouse in her mouth rushing towards her kittens in the bushes. The "Mother Nyah" appears before Torako and the cat hisses at it, standing her ground. Saitama then appears before the cat and defends it from the Mother Nyah's attack. Saitama then grabs Torako and kills the "Mother Nyah". The young girl is happy to see her cat once more. Surprised to see that she had given birth, the girl is suggested to take the kittens with her. Saitama then picks up the cat and notes that the job was harder for him than beating monsters, thinking he might not be fit to be a hero. To this, Genos tells him that he had still defeated the monster which Saitama tells him he was not informed of. Saitama is then scratched by Torako and threatens it and the girl realizes that he is not the kind of hero for normal B-Class work.