Kent Mirai is a spirited boy whose goal is to be the world's top Mushiking champion, and he fills his days with beetle battles alongside Mushiking, the King of Beetles. Then a strange boy named Shiro Mizorogi turns up and overwhelms battler after battler without mercy. Shiro controls a beetle no one has encountered before, the Dark Side Neptune Beetle, cloaked in a sinister aura with eerie glowing red eyes. His dark power comes from Dr. Black, a man willing to do anything to win at Mushiking. Kent and Mushiking must now clash with the Dark Side and its Neptune Beetle. The film is a fantasy adventure based on the popular arcade card game about battling beetles.