In Tokyo, Japan, a man named Dr. Onodera is working on his wharf. Just then, an entity known as the Ancient One arrives and is upset about the wharf that the doctor is creating is a threat to life. He warns him about an incoming threat of a giant monster and disappears.
Meanwhile, in a science convention, Professor Pomfrit has declared his favorite student, Velma, the winner, and her rival, Elliot is not happy about it. She shows her invention: Dogbot. Elliot states that he hates dogs because he is allergic to them, and Shaggy wonders why did Velma created it since the gang already has Scooby. When presenting of how good the robotic dog is, Scooby becomes jealous of it. Professor Pomfrit states that because Velma has won, she and her friends are sent to present Dogbot to Dr. Onodera, a robotics expert.
He tells them that he and Dr. Onodera are studied together years ago in a university of Tokyo. Shaggy becomes sleepy and Fred tells Professor Pomfrit that he doesn't travel too well.
Upon arriving in Dr. Onodera's company, the doctor is impressed by Velma's invention. Suddenly, the lights go out and the Ancient One is angered that Dr. Onodera has ignored the threat. Because of that, he has decided to put an ancient curse to the doctor that he will become a giant monster each night he falls asleep. Just as he is about to do it, Velma orders Dogbot to make a pizza and place the curse there.
Professor Pomfrit asks the doctor what is it about and the reply is that the Ancient One is trying to place a curse on him because he thinks that he is dishonoring Japan by building a wharf. Unfortunately, Shaggy wakes up and eats the pizza, not knowing that the curse is placed there. Dr. Onodera tells him not to worry because the probability of turning into a monster while sleeping is 0.
In the night, Shaggy is about to sleep with his feet being trapped so that when he changes into a monster, he will not go on a rampage which he will not as long as he is awake. However, he suddenly goes to sleep.
However, Shaggy, turned into a monster, goes on a rampage around Japan.
When morning came, Fred, Daphne, and Velma wake up Scooby and Shaggy to have breakfast in bed. However, the latter states that he is not hungry. Just then, an officer named Lt. Keiko Tanaka appears and thinks that Shaggy turns into a monster going on a rampage around town, including the robotics factory. He refuses to believe that because he is bed all night. Just then, his pajamas are shown tattered and the chains are broken by razor sharp teeth, causing Shaggy to realize that he is a monster.
Just then, Scooby and Shaggy are eating noodles in a hut. Velma thinks that Shaggy isn't a monster and there must be a logical explanation for this. Daphne reveals a large footprint and the villagers are starting to go after the gang. Just then, Lt. Tanaka, along with two policemen, arrives to inform Shaggy that because someone has the video of a monster, it is also an evidence for her take Shaggy and the gang into custody.
The gang runs away to a rooftop of a train. Shaggy thanks Dogbot for that action, much to Scooby's dismay. They get inside the train. While Scooby and Shaggy are eating, the gang are watching of how they are declared to be criminals, and on the top of that, Lt. Tanaka has arrived on the train. She manages to grab Shaggy, but because he dropped some plates, Daphne slips up and stops the train with an emergency break.
Afterwards, the gang goes to the downtown of Japan. Suddenly, Elliot arrives and the reason for that is because he made his parents take him there as a consolation price for getting his award taken by Velma. Fred points out that she's not a thief, but Elliot is focused on getting Shaggy and calls the police to get the gang.
While running into the alley, the gang gets into the white sheets and disguises as monks. Unfortunately, Elliot can see through that and Lt. Tanaka grabs Shaggy, but he escapes.
After a chase around Japan, the gang checks in at a hotel for the night. Shaggy states that he wants to sleep in the hotel vault, but the hotel manager refuses to allow that. Daphne has managed to convince him to allow it.
Just as the gang is about to sleep, they find the vault destroyed and find Shaggy as a monster about to crush them with his foot. However, Dogbot has saved them by using a shield. The gang starts follow him.
In the morning, they find Shaggy, whose pajamas are tattered, sleeping in an alley. After the gang states that he is a monster, Shaggy plans on getting himself arrested, but they refuse to let that happen. Velma calls on Professor Pomfrit for help, but what she didn't know is that Lt. Tanaka is listening to their conversation.
The gang arrives in a disguise to get into temple. However, Lt. Tanaka and her men arrive to capture them, but Professor Pomfrit orders Dogbot to make a super size popcorn for the gang to escape. Dogbot has managed to make a chariot for Shaggy. However, he begins to fall asleep.
By the night comes, Shaggy wakes up as a monster. However, Fred, Daphne, and Velma arrive to inform him that they were standing in the model of Tokyo. It turns out that they followed Dogbot to an abandoned movie studio. This proves that Shaggy is not the monster. The gang heads to the robotics factory.
They find Elliot and his parents with Dr. Onodera, leading them to be suspicious of how about the monster. Just then, the monster arrives and chases the gang. Scooby uses Dogbot to give the monster a slip up. Velma uses wasabi to reveal that the monster is Professor Pomfrit.
It is revealed that after he has learned of the conflict between Dr. Onodera and the Ancient One, he uses the legend to frame Shaggy. In order to do that, he has Dogbot follow his commands to get Shaggy away from the action. The reason for this is because Professor Pomfrit wants to become more richer than Dr. Onodera.
When the doctor asks about the Ancient One, Lt. Tanaka states that there is no existing entity like that. Dr. Onodera has decided to build his wharf someplace in order to please the Ancient One. Velma gives Dogbot to Elliot. When Daphne asks her if she's going to miss it, Velma replies that she isn't because she has given it super size brain, something that can't compete with Scooby's super size heart.