Lagoona, the daughter of the Sea Monster, has been looking forward to Spring Break all year and decides to take her friends on an adventure to the Great Barrier Reef. However, their trip takes an unexpected turn as their boat sinks during a storm and they find themselves stranded on a mysterious island shaped like a skull.
The group soon discovers that the island is inhabited by a group of hospitality-driven monsters who welcome them with open arms. However, they soon realize that their hosts are being controlled by a sneaky showman named Farnum, who is using them to capture the rarest, most mysterious monster of them all – the Beast.
Frankie, who is always eager for a challenge, leads her friends in an attempt to rescue the Beast, but she is the one who ends up being captured by the giant monster. However, with the help of her friends, including Lagoona and Clawdeen, she manages to escape and even befriends the Beast.
The tables turn when Farnum and his goons try to capture Frankie and the Beast once again, but this time, Frankie and the Beast work together to escape. They return to Monster High, but soon realize that their adventure has led Farnum right to the school's location, which means that the greatest gathering of monsters is now in danger.
As a result, Frankie and her friends must use all their skills and strengths to protect Monster High from Farnum, his goons, and any other dangers that might come their way. They must also use their newfound friendship with the Beast to ensure that he is not captured and used for evil purposes ever again.