There's a steam monster haunting the boy's shower at Bela Lugosi jr. high. Luckily, the P.I.'s are on hand to stop it. But before they can do that, a new student uses his strange powers to reduce it to a little bite-size steam monster (which he then eats, believe it or not).
The P.I.'s, having witnessed this display, are impressed and want him to join them as the 5th P.I. Not going to happen, though. Instead, this mysterious new student (whose name escapes me) takes them into the netherrealm as his prisoners. It turns out he is a Shadowcop, a sort of police officer from the spiritual world. He has brought the P.I.'s here because they have been charged with crimes against some of the realm's inhabitants (each of whom were beaten by the P.I's in previous episodes).
This won't be a fair trial, unfortunately; the judge is quite intent on making sure our heroes are punished. When the Shadowcop, newly promoted for his role in capturing the P.I.'s, finds out about the judge's bias, he helps the P.I.'s escape. Of course, no good deed can go unpunished and he ends up fighting against the demonic judge along side the Phantom Investigators. Needless to say, his promotion is revoked, but everything works out okay anyway; the judge is beaten and the P.I.'s make it back to the earthly realm safetly.