In the previous episode, Batman and Joker's former henchwoman Venus are chained in a water pit where a giant clam has (almost) already swallowed Robin.

Using every ounce of his strength, Batman bursts free of his chains, rushes over to the clam and pries it open long enough to rescue Robin from the maw of the mollusk.

Batman and Robin fly out to the Jokerjelly-infested Gotham City Reservoir by Batcopter and restore the water supply with the trusty aid of a "Special Exploding Batarang" and the "Portable Batlab".

Batman and Robin return to the Batcave with Venus (having doused her with Batgas, naturally), and after a small tour, they get a surprise when the Penguin, Joker, Mercury, Mars, Saturn, Neptune and Uranus pop right out of the Batmobile's trunk, ready to assassinate them, and convert the Batcave into the headquarters of Gotham City's criminals.

But Batman stops them by activating his newly designed "Batspectrograph Criminal Analyzer", which recorded Joker and Penguin's bone structure, metabolism rate, molecular blood structure, retina patterns, and other invaluable scientific data (he knew they were hiding in the trunk all along, and so he brought them both to the Batcave in order to utilize the analyzer which only works at close range and is too large to move).