OVA 12

Zombieman Murder Case 2 (ゾンビマン殺人事件２, Zonbiman Satsujin-Jiken Ni) is the twelfth OVA of the One-Punch Man anime series.

Summary
Child Emperor, Zombieman, Superalloy Darkshine, and Metal Bat are dispatched to a forlorn, snowy mountain to hunt a monster. They fail to find anything, so they check into a lodge for the night. There they meet Saitama and Genos, who also got stuck on the mountain and had to check in, and Flashy Flash, who is trying to relax on a vacation.

Speed-o'-Sound Sonic is also training on the same mountain, and decides to hunt Saitama once he realizes he is present. However, he stays outside in the blizzard for too long and freezes, losing his balance and crashing through a window. The heroes tie him up, but Sonic still plans to kill Saitama.

Sonic easily escapes the rope and tries to attack Saitama from under the floorboards, but is stopped by Genos, and their encounter destroys the room. The dust irritates Saitama's nose, and he sneezes with enough force to blow open every door and window in the lodge. The power goes out, followed by a sudden explosion.

The heroes grab a flashlight and head to the den, finding the room destroyed and Zombieman unconscious. Child Emperor can't locate any external heat signatures, so he deduces that the culprit must be in the lodge. After recovering, Zombieman tells what little he knows, and Sonic enters the room to prove his innocence. The group finds that they all have good alibis, completely forgetting about Flashy Flash upstairs. They assume the monster must be responsible, but after the Hero Association tells them it is just a bear monster called Bruinado, they decide it can't be.

Once Child Emperor sees Saitama sneeze again, he deduces the following: the abundance of heaters in the lodge dried out the interior, Genos and Sonic's fight kicked up flammable dust, Saitama's sneeze pushed the dust into the den, all the electricity being used caused a breaker to trip, and Zombieman's lighter caused the dust to detonate. The group is relieved at the conclusion.

Suddenly, Bruinado crashes through the front door - however, he is quickly defeated by the abundance of S-Class heroes. Upstairs, Flashy Flash continues to use all the lodge's power as he relaxes.

Trivia

 * Saitama is reading Makai no Ossan (Another one of ONE's works) in this episode.