On a field trip to the aquarium, Arnold can't wait to see Lockjaw, the infamous "terror of the deep." But Lockjaw turns out to be an old tortoise with a graffiti-covered shell--and he needs Arnold's help.
The class are on a field trip, heading to the local aquarium. Arnold says that he has never been to the aquarium before, as he missed his chance when the class went there during 1st grade due to having chickenpox. The class talks about different attractions, but all of them get excited when they hear about Lockjaw. When the bus arrives, the class frantically jumps off and run inside, while Arnold slowly walks in, amazed by the sight.
As the tour guide instructs them on the rules, Arnold asks Gerald who Lockjaw is. Gerald, Sid, and Stinky tell Arnold stories they've heard about Lockjaw's incredible strength, which makes Arnold interested, causing him to go around and ask class members if what they're looking at is Lockjaw, only to have them say what they're looking at is not him. When the class decides to go and see Lockjaw, Arnold quickly follows, the class is eager to see Lockjaw, but when he emerges from the water, the whole class looks in disappointment, due to him looking a lot bigger when they were in 1st grade, Arnold, however, is upset at the state of him, and how he is treated. Harold throws and hits Lockjaw with his ice-cream, causing the whole crowd to burst out into laughter. Arnold, upset at the site, goes downstairs to see Lockjaw underwater, as he feels sorry for him.
When Arnold gets home after the field trip, he gets reminded of turtles, which in turn reminds him of Lockjaw. He goes upstairs, puts some jazz on, and relaxes on his couch, only to have his mind remind him of Lockjaw by having the floor morph into him, and having him float across the sky. Grandma comes in, and Arnold discusses the situation with her, to which she decides that they will take Lockjaw and free him into the ocean. Grandma goes through her closet to find some dark clothes for Arnold to wear when they’re sneaking in the aquarium and she finds a long sleeved red sweatshirt. They drive to the aquarium, sneak in, and upon seeing Lockjaw, decide to clean him up. After they are unable to move him, Arnold gets a skateboard, and uses it to move Lockjaw out to the car, where they drive him to a pier. They roll Lockjaw off the pier using the skateboard, and have him land in the water, where he then swims out into the distance.
Both are proud of what they've done, and Grandma decides to have some turtle soup when they get back home, causing Arnold to sigh.