Sebastian was first introduced during the start of a royal concert occurring within King Triton's palace. The performance is a failure, however, as the star of the musical, Ariel, is nowhere to be found. Sebastian is subsequently faced with humiliation throughout the kingdom, and berates Ariel for this upon her return to Atlantica. King Triton does the same, and further adds to the argument once he learns Ariel missed the performance due to traveling to the surface again. After the princess storms off in despair, Sebastian shows no remorse, feeling teenagers to be too trouble-making for their own good, and should be given constant supervision. This comment gives Triton the idea to assign Sebastian as Ariel's supervisor, tasked with keeping her out of trouble and under close watch at all times.
Sebastian, angered over the new position, heads out of the throne room to find Ariel, eventually doing so and discovering a hidden grotto owned by the princess and used to store various human objects that have sunked into the sea. Sebastian scolds Ariel on the obsession with the human world, and tries to take her home, only to be interrupted by a passing ship carrying the handsome Prince Eric. The ship is soon caught within a hurricane, and Eric falls into the sea, nearly drowning. He is rescued and swam to shore by Ariel, with Sebastian and Ariel's best friend, Flounder, following behind. Horrified by Ariel interacting with the then unconscious Eric, the crab suggests keeping the entire situation a secret, and simply return home and act as if nothing's happened. Unfortunately for Sebastian, Ariel has fallen in love with Eric, and obsessively focuses on him from that moment forward. Sebastian tries to convince the princess that the human world is undesirable through the musical number Under the Sea, but once again, his attempts are futile.
Just then, Sebastian is summoned to the palace by King Triton, who has leaned Ariel has fallen in love (though he believes it to be a merman, as opposed to a human). Triton playfully confronts Sebastian on the matter, wanting to know the identity of Ariel's crush, but Sebastian's anxiety blinds the obvious joy Trition feels, resulting in the former revealing Ariel's act of saving and falling in love with a human. Sebastian is then forced to reveal Ariel's grotto, where Triton destroys most of the artifacts in hopes of getting his message across to his daughter. Sebastian shows remorse for accidentally revealing Ariel's secrets to Triton, and is told to leave the grotto by the princess, which he sadly obliges to. As he tries to explain himself to Flounder, Sebastian spots Ariel leaving the grotto with Flotsam and Jetsam, minions of Ursula, the Sea Witch. Though Sebastian and Flounder try to prevent it, Ariel makes a deal with Ursula to become human for three days; in during which, she must have Eric fall in love with her and bestow a kiss of true love. Otherwise, Ariel will belong to Ursula for eternity.
After the transformation, Sebastian and Flounder aid Ariel to shore, where the former has a nervous breakdown over the situation, making an attempt to return to Atlantica to inform King Triton of what's happened. However, Ariel begs for Sebastian's sympathy, which wins the crab over as he knows Ariel's life would return to misery should she become a mermaid once more. Sebastian then vows to help find Eric, delighting the princess and setting the stage for further events. Eventually, Eric comes across Ariel, and she is taken to his seaside castle with Sebastian tagging along. Sebastian is soon separated from Ariel and comes across the manic Chef Louis who tries to cook the former. Sebastian manages to escape and returns to Ariel, expressing his exasperation over the entire ordeal, only to soften once he sees Ariel's happiness and satisfaction over living above the surface. Over the course of the following days, Sebastian works with Flounder and Ariel's seagull friend, Scuttle, to form a romantic relationship between the princess and Eric, hopefully leading into a kiss. The morning of the third day, however, Sebastian and Ariel learn Eric has found his "true love", and has assembled a wedding to take place that afternoon.
However, Scuttle discovers this new woman is actually Ursula in disguise, urging Sebastian to rush off to Atlantica to finally alert Triton. By the time they find Ursula, she has now taken Ariel (once again a mermaid) as prisoner and offers her freedom in exchange for Triton's soul. Triton agrees and is turned into a polyp, much to Sebastian's sorrow. Just then, Prince Eric arrives to battle Ursula, though Flotsam and Jetsam are sent to capture him. Sebastian and Flounder intervene to free him, while Ariel deals with Ursula, in a scuffle that results in the deaths of Flotsam and Jetsam. An enraged Ursula then turns to a monstrous size to kill Ariel out of revenge, horrifying Sebastian, who can only watch alongside Flounder as Ursula ultimately meets her end at the hands of Eric, reviving Triton.
Afterwards, Triton finally sees the goodness in humanity as a result of Eric's bravery and genuinely selfless love for Ariel, expressing his softened feelings towards Ariel's situation to Sebastian. Triton then magically transforms Ariel back into a human to be with Eric, and the two have a wedding sometime following this event. Sebastian is present at the ceremony, crying tears of joy as he watches the newly weds side by side. The moment is short-lived as Chef Louis returns for revenge, only to be foiled and humiliated by Sebastian's quick-thinking, allowing the crab to triumphantly return to the sea alongside Scuttle, Flounder, and Triton, who sees the happily married Ariel and Eric off.
- His name was written as a text in the Descendants: Wicked World episode, Careful What You Wish For as SABASTAINCRAB1189.
- He was voiced by Samuel E. Wright in his debut and almost reprise his role in later films.