Role in the film
Undertow is summoned by Morgana and is used as a threat to eat baby Melody if Triton doesn't hand over the trident. Morgana's plan to kill Melody is foiled by Ariel, Eric, and King Triton, who turns Undertow into a mini-fish, such as a piranha. Undertow complains to Triton about this, it doesn't work. So, he leaves with Morgana and her two manta rays, Cloak and Dagger.
12 years later, at Morgana's lair, Morgana's attempts to turn Undertow back into a shark fail. After Morgana believes she can't handle the trident's power, Undertow bets that Ursula could have done it. Morgana throws a small glass ball at Undertow (but misses) for criticizing her. Morgana tells her all about her mother criticizing her. Undertow argues with Morgana about whose fault it was that he was miniaturized, whose fault it was about how they all (including Cloak and Dagger) ended up hiding in the icy lair for 12 years, and whose fault it was that they could never show their faces in polite society again. The argument ends with the orb shining and Morgana sees her time for revenge and rivalry against her deceased sister, Ursula, then she will make Undertow into a huge shark again.
Undertow, Cloak, and Dagger lure Melody to Morgana's lair. Undertow introduces Melody to the "one and only" Morgana. After Melody is transformed into a mermaid by Morgana using Ursula's magic, Undertow complains about how he could have been changed back into a huge shark. Morgana calms Undertow by saying: "Keep your scales on, small fry. I'm still reeling her in."
Undertow is next seen with Morgana when she reveals to Melody that the spell is temporary and she tricks her into getting Triton's trident for her.
Undertow appears again when Morgana sends Cloak and Dagger to make sure nothing interferes with Melody's return with the trident. Undertow doesn't go with Cloak and Dagger this time.
Undertow scares Melody's new friends, (Tip and Dash) away. Then, Undertow takes Melody to Morgana's lair. After Undertow tries to rush Melody to give the trident to Morgana, Morgana silences him by snatching him with one of her tentacles. After Flounder (Ariel's fish friend) is accidentally called "Flopper" by Morgana, Flounder corrects her and attempts to attack Morgana but Morgana releases Undertow to attack Flounder. Undertow and Flounder fight. Melody is angry with Ariel for keeping the secret of the locket from her, so she gives the trident to Morgana. Melody learns her big mistakes and Morgana's true colors after Morgana's confessions.
Undertow is chased by Sebastian for taunting Triton. However, Undertow is returned to a bigger-than-before tiger shark by Morgana, using the trident. Sebastian escapes, safely while Undertow gloats.
Undertow finds Tip and Dash trying to save Melody. Dash uses Tip as bait to save Melody while bravely jumping on Undertow and trying to hold him by his dorsal fin. During the chase, Undertow swallows Tip alive. However, Undertow is going so fast and out of control with Dash blocking his eyesight that he accidentally breaks the ice of Melody's prison, leaving him defeated and with all his teeth broken. It's completely unknown what happened to Undertow following this, but it could be possible that he escaped before Morgana's lair sank down, or if that's not the case, he was presumably crushed or dragged with the lair to the depths of the ocean.