The Alden Children are on vacation at an aquarium in Florida, where they feed the penguins, swim with dolphins, and come face-to-face with a great white shark! The shark draws crowds and thrills the park's visitors. But the Aldens soon discover that the shark is in danger—someone at the park wants it gone. How will the Boxcar Children fish out the crook before it's too late for the shark?

Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather, and they're going to a comic book convention!   The Alden Children are searching for a rare comic book of their favorite superhero, Captain Fantastic. When they finally find the comic, they discover something else, too: a strange note inside, "signed" by the comic's creator. The Aldens soon realize the note is fake. But when they start to investigate, the mysterious note is stolen! It looks like Captain Fantastic has a real-life enemy. Who would want to sabotage a superhero? The Aldens will need their super powers of investigation to catch this comic book culprit!