Undertow is summoned by Morgana who is threatening to feed baby Melody to him if Triton doesn't hand over the trident. Morgana's plan to kill Melody is foiled by Ariel, Eric, and King Triton, and King Triton turns Undertow into a piranha. He leaves with Morgana, and her two manta rays, Cloak and Dagger.
At Morgana's lair, Morgana's attempts to turn Undertow back into a shark fail. After Morgana admitting that she cannot break the trident's spell, Undertow bets that Ursula could have done it. Morgana throws a small glass ball at Undertow (but she misses) for criticizing her. Morgana tells him all about her mother criticizing her. Undertow argues with Morgana about whose fault it was that they all (including Cloak and Dagger) ended up hiding in the icy lair. The argument ends with the orb shining and Morgana sees her time for revenge and rivalry against her deceased sister, Ursula.
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 with the help of Ursula's potion, which Morgana had kept secretely from Undertow, Undertow complains about how he could have been changed back into a shark with the potion's help. Morgana calms Undertow by saying "Keep your scales on, small fry. I'm still reeling her in."
Undertow appears again when Morgana reveals the spell is temporary and she tricks Melody into going to get 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 does not go with Cloak and Dagger this time.
After Melody returns with the successfully stolen trident, Undertow scares Melody's friends, (Tip and Dash) away. Undertow takes Melody to Morgana's lair. After Undertow tries to rush Melody to give the trident Morgana, Morgana silences him by snatching him with her tentacle. After Flounder (Ariel's fish friend) is accidentally called "Flopper" by Morgana, Flounder correct 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 realizes her mistake as Morgana, now with the trident in hand, reveals her true ambitions.
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. Undertow gloats.
Undertow finds Tip and Dash trying to save Melody. Dash uses Tip as bait to save Melody. Undertow swallows Tip alive. However, Undertow is going so fast, and out of control. He accidentally breaks the ice of Melody's prison and crashes into a wall. All of Undertow's teeth fall out, leaving him defeated. Tip is freed and he and Dash save Melody. It is unknown whether Undertow got killed or not. However, sharks can grow new teeth after old teeth are lost, so therefore, it is possible that Undertow survived.