"Threat from the Sea" (海からの脅威, Umi Kara no Kyōi) is the 23rd chapter of the One-Punch Man manga series.
A large Mysterious Being rises from the sea and begins to terrorize the nearby city. It declares itself to be one of the Seafolk, and that the humans are to surrender the surface to them at once. His rampage is short lived as Saitama arrives on the scene and dispatches him quickly. Later on, a large wave crashes to shore, bringing with it more Seafolk who then head into the city and start to attack. A citizen is grabbed by one of the Seafolk, but is quickly saved by the A-Class hero Stinger, who then challenges the rest of the Seafolk to battle.
The scene changes to Saitama and Genos relaxing in their apartment. Genos is doing dishes and commenting on their recent rises in rank, stating that Saitama could soon be moved up to B-Class, and mentions that the current Rank 1 C-Class hero has been in his position for more than half a year already. At that point, Genos receives a call from the Hero Association stating that the Seafolk had attacked J-City. He mentions this to Saitama who then turns on the news to check on the situation. The news shows a battered Stinger still facing off against the Seafolk. Saitama, now serious, states they need to get there as fast as possible.
An exhausted and battered Stinger stands before several large Seafolk. They tell him to give up, that all his supporters had left and that he was going to die there. Stinger blows off these jabs and states that now that he is by himself he can go all out. One of the Seafolk sends an attack at Stinger who meets it with an attack of his own that blasts a hole through the Seafolk. Leaping through the whole he uses another attack and dispatches the last three Seafolk. He lands and starts to celebrate his victory only to be immediately taken out by the Deep Sea King.
D-Pad is driving toward J-City with his truck along with Funeral Suspenders, Studless, Gasmask Cowboy, Hyottoko, Food Battler Futotsu and many other C-Class heroes, when they were suddenly outrun by Saitama and Genos.