Brain Juice begins with a prologue that introduces the two aliens. Looking for slaves to bring to their emperor, Morggul and Gobbul have remained hidden on earth within the home of Dr. Frank King. While they originally think he might be the one to capture, they change their minds once they see Nathan and Lindy Nichols visiting their Uncle Frank.
As their emperor demands young, intelligent slaves, they seem to be the perfect fit. The only thing the pair of aliens need to do is slip them a dosage of Brain Energizer Fluid, and they will be ready for capture. Unfortunately for Morggul and Gobbul, Nathan and Lindy have a reputation for being the stupidest kids in their school, and they have come to their Uncle for help in boosting their intellect. Thinking fast, he gives them what he labels "Brain Juice", secretly just grape juice in disguise. Unbeknownst to the doctor, Morggul and Gobbul replaced the juice with their Brain Energizer Fluid. Soon, the childrens' intellect problem is solved, as they go from stupid to geniuses.
Eventually, Morggul and Gobbul capture Nathan and Lindy and bring them to their home planet. However, upon arrival, it seems the two are losing their intelligence, forgetting what even basic words mean. Morggul and Gobbul don't buy it, but the emperor is displeased by the seemingly unintelligent slaves. As punishment, the emperor disintegrates the pair of aliens.
Morggul and Gobbul are similar in personality, with some differences. Gobbul appears to be the leader of the two, but his partner often scolds him for his lack of patience. Both Morggul and Gobbul look down upon the people of Earth, finding them repulsive. They agree that the Earthlings are primitive creatures, lacking the intellect and complexity that their species does.
The two aliens are described as having a bell-shaped body with green, wet skin, and yellow eyes. Each has four tentacles and a pair of webbed feet. Their heads are described as froglike, with cruel, ugly faces, and multiple mouths.
Gobbul is revealed to be the taller of the two, and has a pair of silvery tusks, much like a walrus.