One night while piloting his boat out at sea, Bruce Wayne is forced to abandoned ship when it hits a rock and starts to sink. Swiming to a nearby island, Bruce saves Mark Desmond and returns him to his brother Roland.
Months later, Batman and Robin begin investigating a giant creature dubbed "The Blockbuster" that has been terrorizing and robbing various locations in Gotham City.
Following the massive creature back to the island that Batman recognizes that the one the Desmonds live on, Batman deduces that Blockbuster is really Mark Desmond. While appealing to the beasts human side by changing to Bruce Wayne and calling for help, Robin captures Roland.
Taking Roland to shore to answer for his crimes, it is learned that Mark had taken a serum to become the Blockbuster in order to be strong, and that Roland exploited the monsters strength. Going back to the island to collect Blockbuster, while disguised as Roland, Batman finds that the monster has walked off into the ocean.