Kermit, Evan's cousin, takes the can from Andy while she is showing it to Evan. When Kermit opens the can, the boys see blue Monster Blood inside. Andy quickly closes the can. The kids see Conan Barber, the local bully, and Kermit uses the blue substance in the can to taunt him. Kermit accidentally tilts the can, causing some of the blue Monster Blood to fall out. The substance forms into a blob-like creature with eyes. The creature races over to the garden hose, and it drinks until it bursts into two creatures. Kermit attempts to trap the creatures by luring towards a puddle of water, but they get to the water and multiply into four — seemingly angrier — creatures. Eventually, Evan catches the creatures in a plastic bag.
For the rest of the book, the kids try to contain the creatures and stop them from multiplying. Each time they multiply, they seem to get meaner and meaner.
In the end, the creatures end up defeating themselves; they become so ferocious, they begin attacking each other. When the kids meet the creatures' creator, they learn that the original substance wasn't even Monster Blood. The blue creatures are the result of ten years of work and $50 million of investment in development of an experiment for an underwater fighting force for the army. In the book's twist ending, it is revealed that anyone who eats the blue substance from the can will become incredibly thirsty, and they will also multiply involuntarily after drinking liquids.
The first blob that Evan lets out acts innocently. However, once it divides into two creatures, each blob begins acting meaner. This trend continues, and the blobs get meaner with each iteration.
Unlike the original Monster Blood, this substance is blue. Each of the blob-creatures is small, blue, and slug-like with black eyes. According to the cover art, they have eyestalks and large lips.