Lagoona Blue and the Big Sea Scarecation is the third book in the Monster High Diaries book series.
Monster High is about to close for Spring Break, and Lagoona is excited about spending the week with her family. Gil invites her and her family to come with him to the resort he and his parents are going to, but Lagoona is adamant about going home and chilling out at her home in the Great Scarrier Reef. But when she hears about the exciting holidays her friends have planned. she feels a little let down about her plans being the same old thing. The next day, Lagoona returns home and meets her siblings and parents who have a surprise for her - The family is going on a luxury cruise ship for the week.
Upon boarding the ship, Lagoona and her siblings start having fun on the ship's various swimming pools. While looking for other monsters her age, Lagoona is surprised to see Toralei is aboard. It's not long before Toralei is up to mischief, convincing the lifeguards to hold a volleyball game in the pool, preventing Lagoona from swimming. Lagoona decides to not look bothered by it, and joins in the game.
The next day, Lagoona takes her siblings to the surf pool to see them how to surf. She sees Toralei and, deciding to be friendly, offers to teach her surfing. Toralei seems interested for a moment, but then Lagoona's brothers accidentally knock her into the pool. Believing it was done on purpose, Toralei vows revenge, and spends the rest of the day preventing Lagoona from enjoying the pool, including dumping live lobsters into the water.
That night Lagoona joins her family for dinner, where her father informs her of a scuba excursion to some nearby reefs. Toralei stops by, and tells the family that there will be a singing contest for families, and convinces the Blues to join up. Lagoona, who is terrified of performing on stage, tries to get out of it, but Toralei informs them that the whole family needs to join to be eligible to win. Not wanting to let her family down, Lagoona reluctantly agrees.
The next day, the Blues head out the scuba expedition to the reefs. But while exploring, a storm rolls in, and the captain orders a return to the ship. Lagoona swims out to collect her siblings, and she and her sister Kelpie get a fright when they sees something strange under the water. But it turns out to be a family of jellyfish creatures, who were keeping an eye on Lagoona's brothers.
Everyone returns to the ship, where the deck is closed off due to the storm. The Blues try to relax in the ballroom, where Lagoona sees Toralei huddled in a corner. Guessing that she's frightened of the storm, Lagoona decides to comfort her. Toralei admits she's scared the ship might sink in the storm, but Lagoona assures her the ship is safe, and invites her to help her family with their song for the talent contest. Toralei accepts, and apologizes for all the pranks she pulled. As Toralei tries to teach Lagoona's brothers some dance moves, Lagoona sees that Toralei likes to be in control at all times, perhaps its why she likes to play pranks on others.
As the talent show draws nearer, Lagoona admits she's afraid to perform on stage, and Toralei suggest that she take Lagoona's place in the performance. She admits she made up the ruse about the whole family needing to participate just to get a rise out of Lagoona. She performs with the Blue family, and they win the contest. The next day, Lagoona gets a surprise when she discovers Toralei has convinced the captain to stop at a nearby island, which happens to be the resort where Gil is staying. As she spends the day with Gil, Lagoona wonder if maybe she and Toralei are becoming friends, or if it's just the calm before a storm...
The story makes some references to Great Scarrier Reef, including Kelpie's fear of the Kraken and Lagoona's animoscity towards Toralei, suggesting the story takes place before the events of the movie.