Chum is first seen in the film when Bruce brings Marlin and Dory to the meeting. He is seen swimming around in circles with Anchor and having a shoving contest with him as Bruce approaches. He complains that him and Anchor have already gone through the snacks and he's still starving. The next scene shows Chum at the meeting saying the pledge, although he recites it in a monotone and looks somewhat bored. He becomes more lively when Anchor mentions that dolphins are not their friends, he then says that dolphins think they are so cute. He then does an impression of a dolphin, complete with a "dolphin clap" which earns him a laugh from Anchor.
However, Bruce then moves on with the meeting and announces that the meeting was step five: bring a fish friend. When Bruce sees that Anchor had his friend, he asks Chum where his friend is. Chum then gets a suspicious look and claims to have misplaced his friend and lets out a smile only for a fish skeleton to pop out of his mouth. He quickly sucks it back in and gives Bruce an innocent smile. Bruce says that it was alright and knew that it would be a difficult step and offered Chum one of his friends. Chum grabs Marlin and hugs him under his fin, much to Marlin's discomfort. As Bruce continues to talk, Marlin tries to leave the mako's grasp only for Chum to pull him closer. When Bruce announces that he had not eaten a fish in three weeks, Chum begins to clap his fins, hitting Marlin numerous times as he did so. Once Dory went up onstage and announced that she had never eaten a fish, he, as well as Bruce and Anchor, were stunned at first, yet Chum broke the silence by clapping and telling Dory, "That's incredible!"
When Bruce turned to Marlin to ask whether he wanted to go onstage, Chum gave Marlin a rub with his arm...er, fin-pit. After Marlin was pushed up to the podium he announced that he was a clownfish which excited the sharks. Chum got so excited that when he jerked up he ended up hitting his head on one of Anchor's eyes. When he got close to Marlin he was pushed back by Bruce and hit a metal object which resulted in a small "clang." He is able to shove his way in between his two friends telling Marlin that he loves jokes. Marlin agrees and starts to tell them a joke. However, as Marlin starts to rant, Chum loses interest, he then thinks that the joke's punch line is "Nemo", causing him to burst in a fit of laughter and tries to get Bruce to laugh only to stop short and stating, "I don't get it." He then learns that Marlin was talking about his son, who was taken by divers. Chum then says that humans think they own everything. He is then shocked and shares a sympathy look with Anchor when he finds out that Bruce never knew his father, and as he gives his friend a hug, his words of advice are, "We're all mates here, mate."
As Bruce starts to calm down, Marlin and Dory reappear with a diver's mask which they start to fight over. When Marlin accidentally hurts Dory she starts to bleed causing Bruce to go into a feeding frenzy. Chum with the help of Anchor manages to subdue Bruce for a short time before the great white throws him off. As Bruce chases after the fish, Chum calls for Bruce to remember the steps, he then chases after him only to get stuck in an escape latch with Anchor. Finally, before the end of the chase, he apologizes to Marlin and Dory about Bruce's behavior. Chum was also the one to call Bruce's attention to the torpedo that was about to hit a live mine.
His fate remained unknown throughout the movie until the end where he showed up at the reef, dropping off Dory from a meeting and telling her to keep up with the program before swimming off.
Chum appears in one of the Finding Nemo computer games. In one, you can click on him and he will say a few phrases. In the game, it is revealed that he is the one having the most trouble with the program. He states that he can't stop eating fish, which may explain what happened to his "friend" in the film.
- In the Finding Nemo essential guide and sticker book, it is hinted that he hangs out with at least three different groups. The first one is the FEA which includes Bruce and Anchor, the second is a group of tough sharks that nickname him "snaggletooth", and the third is a group that includes posh sharks and refers to the former two groups as "reef-raff."
- He has a souvenir fishing hook lodged in his snout from a tussle with a fisherman.
- He seems to be the most hyper out of all three sharks. This would be fitting his nature, given that mako sharks are the fastest of all shark species.
- Like his two other friends, he's shown to be able to stay still in water, despite his actual species being unable to do it in real life.