Prison

 (ぷりズン, Purizon) is an extra chapter included with Volume 4.

Summary
Sonic is seen being restrained by various special equipment invented by the hero association as the security guards bring him to his cell. Sonic is brought to the "Smelly Lid Prison", a detention facility for the most vicious criminals. The senior inmates attempt to have some fun with Sonic, but quickly realize that they have underestimated Sonic. Sonic declares to the other prisoners that he will kill all of them to make his stay at the prison enjoyable. Angered, the other prisoners start to fight Sonic one on one. However this quickly angers the boss of the prison and all of the prisoners, except Sonic, try to better their behavior. It is then revealed that it was the boss of the prison that brought order to the prison and forced the prisoners to cooperate and behave. The eldest prisoner tells Sonic that Pri-Pri-Prisoner is a pedophile and turned himself in to the prisoner in order to "legally" harass boys. After the eldest prisoner's lecture, Pri-Pri-Prisoner finds out the Stinger is in danger on the news and Pri-Pri-Prisoner decides to go and save him. Pri-Pri-Prisoner escapes the prison by making a large hole through the wall. Sonic uses the hole to escape as well. The boss of the prison is finally revealed to be Pri-Pri-Prisoner. Saitama and Genos are seen watching the news and they decide to go to J-City.

Characters in order of appearance

 * 1) Sonic
 * 2) Demolition John
 * 3) Pri-Pri-Prisoner
 * 4) Stinger
 * 5) Saitama
 * 6) Genos