Attack of the Mutant, also known as Attack of the Masked Mutant, is the twenty-fifth book in the original Goosebumps book series. It was first published in 1994. The illustration on the cover shows the Masked Mutant outside of his headquarters.
Skipper Matthews has an awesome comic book collection. His favorite one is called The Masked Mutant. It's about an evil male super villain who's out to rule the universe. Skipper can't get enough of The Mutant. Until one day he gets lost in a strange part of town and finds a building that looks exactly like the Masked Mutant's secret headquarters. A building that appears and disappears. Has Skipper read one too many comic books? Or does the Masked Mutant himself really live in River View Falls?
Skipper Matthews is an overweight twelve-year-old comic book collector who loves collecting many comics yet only reads one: The Masked Mutant. The Masked Mutant is an evil male super villain who can change his molecular form into any solid material. He is constantly opposed by a league of superheroes lead by the Galloping Gazelle, the world's fastest man. Skipper has a friend named Wilson who is into collecting rubber stamps and attempts at various times throughout the novel to share his passion with Skipper to no avail. Skipper also has a little nine-year-old sister name Mitzi who always likes to mess around with him.
One day while riding the bus to the dentist, he meets a pretty redhead girl named Libby (unaware to Skipper, she was actually the Masked Mutant in disguise), who strikes up a conversation with him about comic books. He gets so caught up talking to her that he gets off on the wrong stop and spots a building that looks exactly like the Masked Mutant's headquarters. He almost goes inside the building, but he is late for his dentist appointment and decides to come back. The next day he comes back to see the building but it has disappeared. Later that night he goes home and reads the newest issue of the Masked Mutant that has arrived in the mail for him and sees that in the comic book the Masked Mutant has put an invisibility cloak on the building.
Going back the next day, Skipper runs into Libby for a third time. The two decide to see if they can access the invisible building and sure enough they enter successfully. When they arrive, a yellow light ray scans over Skipper's body but has no apparent effect on him. The two ventures into one of the building's elevators and are whisked down to the basement of the building. The two get separated and while alone Skipper finds a large printing press and layouts for the last issue of the Masked Mutant. Skipper concludes that the building is likely the headquarters for the comic book publisher. While leafing through the layouts Skipper notices a series of panels for the next issue, and the new character drawn within looks exactly like Skipper. At this point, Libby reappears and makes Skipper leave with her. That night, Skipper gets home and there is another new issue of the comic waiting for him.
Inside the issue Skipper spies drawings of himself walking around the headquarters with the words "A NEW FOE" written about him. Skipper also finds out that the Galloping Gazelle (the Masked Mutant archenemy) has been held hostage in the Masked Mutant's headquarters, and that only "the boy" can save him.
So Skipper returns again to the headquarters and makes his way up the elevator to rescue the Galloping Gazelle! He finds the bound superhero tied to a chair in an unlocked room, unties him, and the two leave the room to confront the Masked Mutant. The Galloping Gazelle is convinced that Skipper is a superhero and keeps drilling him for his secret power as they make their way to the Mutant's private offices. The Galloping Gazelle informs Skipper that his plan is to run around and around the Mutant at such a fast speed that he becomes a cyclone and sucks the Masked Mutant into submission. Suddenly the Masked Mutant morphs out of hiding (he was disguised as office furniture) and the Gazelle makes good on his plan. However the Mutant outsmarts him and sticks his foot out, tripping the Gazelle. The Masked Mutant then transforms into the form of a leopard and tries to eat the Galloping Gazelle.
The Galloping Gazelle proves himself to be a coward and weakling as he escapes the Masked Mutant's leopard clutches and abandons Skipper with the Masked Mutant as he makes his escape. The Masked Mutant returns to his normal form and he too starts to drill Skipper about his secret mutant-power. The Mutant lifts Skipper all the way up to the ceiling of his private office and is about to drop him to his death when Libby appears in the doorway.
The Masked Mutant safely sets Skipper down as he goes to confront Libby. Libby removes a yellow toy gun from her satchel and tells Skipper that if this is really a comic book then anything can happen, so she tells the Masked Mutant that the toy gun is a Molecule-Melter. The Masked Mutant doubts her and he advances as she pulls the trigger. The gun fires at the Masked Mutant and the Mutant melts down into nothing. Skipper is overjoyed and Libby tells him of course it worked, after all, it is a Molecule-Melter, and Skipper is the next victim and target.
Skipper then watches as Libby transforms into the real Masked Mutant. It turns out that the previous Masked Mutant was actually the Magnificent Molecule Man, who worked for the Masked Mutant. What else matters is that, there is no girl named Libby at all after the Masked Mutant himself ditches his female disguise. The Masked Mutant also informs Skipper of another dastardly deed. He has turned Skipper into a comic book character, which turned into a big shock. The mysterious yellow ray when he first entered the building was a scanner and it scanned him into the comic book. With no hope of escape the Masked Mutant prepares to destroy Skipper once and for all until Skipper stops him by announcing that he is not actually Skipper. No, he is the Colossal Elastic Boy in disguise. The Masked Mutant shakes his head and says, "I knew it!" Skipper reveals that nothing can harm him, but sulfuric acid. The Masked Mutant transforms into a wave of acid, but Skipper barely misses the wave and the acid eats into the carpet.
Skipper made up the whole thing about being the Elastic Boy. Since the Masked Mutant could only transform himself into solids, Skipper had tricked him into transforming into a liquid and thus the Masked Mutant was unable to return to his original form. Skipper was very happy to be alive and performed a little victory dance, on the carpet. He could not wait to return home and see his family again. It seemed like the bus ride home took hours, but as soon as he got home and walked into the front door, he was glad to be back. He had enough adventure for one day, and decides not to read the new comic that has arrived for him in the mail. "Who needs it?" he asked himself about the comic. He then, celebrates his victory later on that night, by playing Frisbee with his little sister, Mitzi for a half an hour. They had a great time. Then Skipper asked his little sister if they wanted a snack. She said yes, and they decided to have some chocolate cake together. As Skipper is cutting the cake with a big cake knife, he accidentally cuts the back of his hand. But he discovers that it was not blood coming out...he was bleeding red, blue, yellow and black ink. Looks like Skipper's comic book career is not over yet and now he must read the next issue of the Masked Mutant comic series in order to continue.