OVA 6

"The Murder Case That Is Too Impossible" (不可能すぎる殺人事件, Fukanō-sugiru Satsujin Jiken) is the sixth OVA of the One-Punch Man anime series.

Summary
After the Dark Matter Thieves failed invasion the heroes that participated were invited to the hot spring resort, when Zombieman is found murdered on the floor. Child Emperor states one of the S-Class heroes is the culprit. The scene then flashbacks the day before the murder had happened which all heroes are invited to stay overnight at the resort. The heroes enjoyed themselves in the hot spring. Metal Bat wasn't satisfied that some of the heroes like Flashy Flash, Pig God, Watchdog Man, that did not participate in the battle, got invited as well. As they were also dealing with other monster cases, they get certain rewards as well. After the hot spring, Sitch gathers all the heroes into a celebration of the victory against the invasion. Few heroes became drunk, after drinking the sake and disgusted Child Emperor, who is later asked by Zombieman whether or not he's enjoying himself. The scene returns to the murder. Child Emperor gathers some of the evidence and tries to eliminate one and another to find out the truth. One of them being the weapon which stabbed Zombieman which is Atomic Samurai's sword. He then interrogates a few other heroes including Saitama, which asked Zombieman if he is truly immortal and finally asked King for more information. King tells him a brief moment of his witness and shows a menacing face which scared Child Emperor a bit. Suddenly, Tatsumaki burst out of the door and is about to restrain everyone for being suspects of the murder when Zombieman wakes up. He then reveals that Atomic Samurai's katana was floating with green light surrounding it similar to Tatsumaki when she is flying. Child Emperor then interrogates Tatsumaki what was she doing last night and predicted that she was drunk for all that sake. She angrily opposes that she knew Zombieman cannot be killed and wouldn't do such things. Bang steps up and calms her down and that everything's fine now that Zombieman is fine once again. Everyone then disbanded. Sitch then asked Child Emperor to look at a footage of a surveillance camera of last night. It shows that Tatsumaki is the culprit and she was very drunk. When walking home, Saitama was wondering why would Tatsumaki stabbed Zombieman. Genos knew it was Saitama's fault but doesn't tell him. It reveals that Saitama told Tatsumaki that she is a child and shouldn't drink Adult sake. Feeling humiliated, Tatsumaki takes revenge by drinking as much as she could from Saitama's bottle of sake.