It's a dark and foggy night, when a mailman is in front of a huge house. He knocks on the door, saying that he has a package for a Professor Grier. He knocks a few more times, only to realise that there is no one home, when he hears a creepy moise coming out from the bushes. Suddenly, a huge humanoid Mantis Creature appears and tries to catch him, but fortnuately he is able to get into his van and run away.
Meanwhile, the Mystery Inc. is on their way to visit one of Velma's former teachers, Professor Grier. Scooby and Shaggy try to find out where exactly is the location of the professor's house, when they find it: Vulture's Claw Botanical Gardens. The name spooks the duo, and they get even more scared when the van of the mailman, who had been able to get away from the Mantis Creature, almost crashes onto the Mystery Machine. After a few more misadventures, they all end up in the entrance of the Botanical Gardens, which is a gloomy and creepy place, and where they meet Mr. Keeble, the Garden's janitor who tells them that there's something creepy lurking around and that they should leave and take the professor with them. The gang decides to find the professor, and figure out what is really going on. While crossing the garden, Scooby and Shaggy run into the Mantis Creature who tries to get them, but they run away, joinging the rest of the gang, who crossed paths with Professor Grier.
A little later, already inside the Professor's mansion, the gang asks him about what is going, so he shows them that he has found a hidden laboratory of what he suspects to be a mad scientist's laboratory, where there were conducted experiences that changed a man into a preying mantis humanoid creature, according to a diary he has found, where all kinds of bizarre experiences were registered. Hearing this, Scooby runs out, only to clash with a man who had been hearing the conversation. This one is revealed to be Dickerman, the real-estate agent who wants to purchase the Professor's house and the Botanical Gardens. Professor Grier states that he is not interested and tells Dickerman to get out of the premises. After Dickerman exits, he tells the gang that he still does not own the property, and that Dickerman is making efforts to but it, but tell them that they must not worry about that and that they should leave in the morning, before anything bad happens to them, with the creature lurking around.
Velma says that they cannot let Professor Grier alone in that house, and that they should try and convince him to go with them, but Shaggy says that if he wants to stay, that's his business, not theirs, and that they should get some sleep and talk about the matter in the morning. While Shaggy and Scooby get eady to sleep, Fred and Daphne, joined by Velma, decide to go back to the laboratory and see if they can find any clue. They invite Shaggy and Scooby to go with them, but they decide that it is best for them to stay in their bedroom. They eventually regret not accepting the offer, and decide to go after them. Unfortunately, when they are about to exit the room, the Mantis Creature appears and tries to grab them. They are able to get a slip on the monster, but only for a few moments. Pursued by the Mantis Creature, the cowardly duo tries to find a way to get out of that mess. After awhile, they end up in what looks to them like a cave, where they meet a hungry crocodile, who tries to eat them.
Meanwhile, back at the laboratory Fred and the girls find out that the professor has an assistant called Eric Arby, who is also looking around for clues, trying to solve the mystery. The four of them end up discovering that most of the equipment in the laboratory is not even plugged, and that the records in the diary seem to be rather incomplete, when Shaggy and Scooby appear through a secret-passage that connected the cave in the laboratory itself, with the crocodile still on their feat. Finally, when these two are able to calm down a little, they all hear the Mantis Creature roaring, but they do not know where the roar is coming. Eric decides to get out of there, and disappears. The Mystery Inc. goes after them, and in the process, Daphne gets kidnapped by the Mantis Creature.
The gang, knowing that the hallways must be crawling with hidden passages and secret doors, decide to awake the professor, so he can advise them on what they should do. Just as they are able to wake him up, Shaggy notices that there is something or someone in the gardens, and Fred decides to check it out, believing that it could be the monster. The professor tells them that he will join them, as soon as he gets dressed.
Already on the garden, Fred, Velma, Shaggy and Scooby are able to catch the person who was lurking around, Dickerman. He tells them that they should mind their own business and disappears. Scooby and Shaggy still try to grab him, but instead of grabbing Dickerman, they grab the Mantis Creature. Scared by the roars, the gang runs into a cabin and lock themselves inside. They hear someone knocking on the door, and Shaggy tells them to not open. But Velma fins out that the sound is not coming from outside, but rather from inside. She finds out that it was Daphne who was making the noises. While untying her, Mr. Keeble enters in the cabin and warns them that should really get out of there, to which Fred answers that they are not getting out of there, before they find out what is really going on.
While going back to the mansion, the gang tries to put the pieces of the puzzle together, not realizing that they are being watched. When they enter in the mansion they find out that have been stepping on something, and that something is oil. The professor joins them, telling them that he was not able to join them sooner becausewe could not find his coat. Velma tells him that they have solved the mystery, when they hear the Mantis Creature roaring, and see the monster's shade in the window. Scared by it, the professor runs away, and Shaggy and Scooby are about to follow right after him, when velma notices that the professor left a trail of oily shoeprints after him. She tries to rationalize that last clue, when the Mantis Creature appears in the lobby and tries to get them. Scooby and Shaggy improvise a plan to get him, before they hurt Fred and the girls. They decide to drop a chandelier on top of the creature, immobilizing it. After that, they try to find out who's the culprit behind the mask, and end up finding out that neiter Mr. Keeble, nor Dickerman, nor Eric are the ones behind it. The person behind the mask was none other than Professor Grier himself. Velma deduces that the professor was trying to scare away Dickerman from buying the property. He intended to use the oil he had found to restore the Botanical Gardens to their former glory, but to do that he had to come up with a plan, so he created the legand of the Mantis Creature to scare everyone way.