While three of the team members prepare for graduation, Jamal's cousin Casey is writing a story for a contest involving a purple slime monster doll that she recently received. The Ghostwriter Team, along with GW, all want to lend a hand in helping Casey write her story before the deadline arrives. The story is funny, just like Casey, full of "gotcha" jokes and other types of pranks. Later on, it seems the slime monster doll, aptly named "Gooey Gus," has a mind of its, er...his own. The team's fears become realized when Gaby and Casey hunt down the doll following a slime attack on Gaby and pour water on the toy. He grows to human-size. Gaby and Casey having realized too late that the water was hot due to laying out in the sun too long, and now the team must really race against time to stop the attack of the Slime Monster!
Will the team finally put an end to Gooey Gus, before more innocent people become entangled in his slimey grip?
This was the last episode produced for the series. Until less than two years later a short-lived sequel series called The New Ghostwriter Mysteries was produced, which only lasted one season because of poor ratings.