Goosebumps : Attack of the Mutant

Skipper Matthews has an awesome comic book collection of which his favorite is 'The Masked Mutant'. Skipper can't get enough of the Mutant until one day, he gets lost in a strange part of the town and finds a building that looks exactly like the Mutant's secret headquarters. A building that appears and disappears, has Skipper read one too many comic books or does the Masked Mutant really live in Riverview falls?