Saitama is a hero who only became a hero for fun. After three years of “special” training, though, he’s become so strong that he’s practically invincible. In fact, he’s too strong — even his mightiest opponents are taken out with a single punch, and it turns out that being devastatingly powerful is actually kind of a bore. With his passion for being a hero lost along with his hair, yet still faced with new enemies every day, how much longer can he keep it going?

承受着残酷命运的少女露恩•芭洛特（林原惠美 配音）与强敌鲍伊德（矶部勉 配音）进行着实力悬殊的惨烈战斗，危急时刻，号称万能兵器的犯罪搜查官乌夫库克博士（八岛智人 配音）出现，拯救了濒临死亡的芭洛特。当芭洛特再次醒来，发现自己正置身于传说中的禁忌科学诞生之地——“乐园”实验室，再次她见识了许多奇怪的发明，并了解到“乐园”和三博士的过去。经过一番追查，她终于得知榭尔（中井和哉 配音）的记忆正藏在卡西诺赌场的一枚筹码之中。为了追查榭尔的过去，芭洛特化装成赌客走入凶险的赌场…… 本片根据冲方丁的科幻小说改编，为“壳中少女”三部曲中第二部（另两部分别为“压缩”和“排气”）。