What a shock: there's a green ghost in Tom's cellar! Slimy Hugo is an ASG, an Averagely Spooky Ghost, and Tom soon realizes that Hugo is not only completely harmless, but also desperately needs his help. Hugo cannot go back to his haunted house, because a dangerous AIG, an Ancient Ice Ghost, has moved in and is spreading an arctic cold over the entire town in the middle of summer. Tom and Hugo go to professional Ghosthunter Hetty Cuminseed, who doesn't like children or ghosts very much, and who just lost her job at the CGI, the Central Ghosthunting Institute. Hetty teaches Tom and Hugo the basics of ghosthunting and the three become an unusual team: only with friendship, courage and self confidence can they overcome their adversary and save the town from the AIG.