Fungus is worried - he has been spotted by a human ("dry cleaner") on one of his trips up top. Meanwhile, he and Mildew are despairing of their teenage son Mould. He is starting to become a "drop-in" - washing regularly, even using deodorant! Something has to be done to rid Mould of his fascination with the Dry Cleaner way of life. Fungus decides that he will, for the first time, take Mould, "up top". Maybe a dose of the reality will cure Mould of his unhealthy obsession? Up top, after a very successful night of scaring, during which father and son really begin to bond, Fungus and Mould are spotted at dawn by Jessica, a dry cleaner intent on proving to herself that she didn't imagine what she had seen in her kitchen a few days ago! During the panic that ensues from them being spotted, Fungus and Mould get separated!

Fungus returns home in shame - he has lost Mould up top. But Mould suddenly re-appears - he has found his own way home. Unfortunately, he was spotted by Jessica, who grabbed him as he fled through the Bogeyhole and was dragged into Bogeydom with him. This is the worst crisis Bogeydom has ever seen - a dry cleaner discovering the existence of Bogeydom. As Fungus' family panics about how they can conceal Jessica, Mildew's sister Septic grows suspicious. But when she and Gunge confront Fungus, all are amazed to find that the room where they thought Jessica was held is empty

Mould and Jessica become friends. But Jessica, with the unwitting assistance of Mould, manages to escape back home. Now Fungus and Mildew must create a plan of damage limitation. They determine that Fungus and Mould shall go back above ground to find Jessica, make sure she hasn't told anyone and if she has, either bring her back, or block up the Bogeyhole. Meanwhile, an exhausted Jessica has told her parents about her adventures, and her father George, sees this as his chance to advance his stagnating newspaper career to get the big story. He is determined to catch a Bogeyman...