In the Characters' Past
On board the Jolly Roger, a young Killian is consoled by his father, Brennan, who promised his son that he would become the person that he was meant to be, only to suddenly abandon him and his brother, as he sold his sons into servitude, because Brennan was a wanted man and the ship was stolen. Years later in The Enchanted Forest, Regina recruits Hook to kill Cora, and takes Hook to a tavern, where she requires Hook to kill his father, who is now the tavern's owner. After the tavern closes, Hook confronts Brennan, who explains that his attempt to flee the law failed and was put under a sleeping curse as punishment; Brennan fell in love with the sound of his nurse's voice and she with him, allowing a kiss to wake him. They married, but she died soon after having their son. Believing that Brennan has changed, Hook has to make it look like he had his father killed, so he can help his father and half-brother escape. When Hook follows Brennan back to his home, he watches him with his son, and witnesses his father tell him the same things he told Hook when he was little, which makes Hook very enraged, thinking that his father lied about having changed. Hook then hears his father address his new son as "Liam". When Brennan goes outside to meet Hook, he takes back his offer, telling Brennan that he thinks he can replace Liam and continue making false promises, though Brennan claims that he named his new son to honor Liam's memory. Despite Brennan's insistence that he had changed, Hook kills Brennan, making Hook the person he is now. As Brennan dies, however, he tells Hook that it's never too late to change, to become a better man.
In Storybrooke
Hook, remembers the last time he saw his father, when Gold arrives to confront him but Hook is steadfast in carrying out his plan. Meanwhile, Emma is determined to stop Hook and everyone follows her and tells the others to split up. Regina and Robin then run into Zelena, who vows to get custody of her daughter, only to be confronted by a group of Dark Ones who then place a mark on Emma's family and friends in a similar fashion. Gold explains to Emma that it is the Mark of Charon, who navigates the ferry to the Underworld. The Dark Ones plan to sacrifice the souls of the marked living, who will replace them in Hell. Then, the 21 resurrected Dark Ones march through Storybrooke, led by Nimue, to prepare for their final battle.
Regina confronts Hook at the docks to remind him about what happened back in the Enchanted Forest, but Hook chokes her, saying times have changed. At Granny’s, Mary Margaret, David, and Henry choose to spend their last hours alive, while Emma reveals her plan to Regina that she wants to sacrifice herself of the darkness inside. Gold then hands over Excalibur to Emma and arranges for Belle to escape town and live her life. Regina and Robin then confront Zelena again ready to take full custody of her and Robin's baby and everything Regina owns once they both die with the others. However Robin tells Zelena that their daughter deserves her best chance with Regina also stating that it's not with her, so she uses Merlin's Wand to conjure up a magic tornado to send Zelena to Oz, with Zelena vowing to return. Emma prepares her plan to confront Hook, who appears at her house, but he has the upper hand after he transforms into Henry, fooling her into handing over Excalibur, which leaves her defenseless.
The 21 past Dark Ones, led by Nimue, bring the residents to the lake that will take them to the Underworld, and act as a barrier to keep them from escaping. Hook appears to help Nimue carry out the plan as Emma is left to watch. Regina then reminds Hook that he is unable to destroy Emma's family because he has changed as a person, and urges him to choose what kind of person he wants to be. Emma attempts to stop the Dark Ones from sacrificing her family, but Nimue starts choking Emma, despite being unable to kill her, to keep her from interfering. As Hook watches, he realizes the truth behind what Regina said, and uses Excalibur to absorb all of the darkness from Nimue and the Dark Ones into himself, which also traps all of the past Dark Ones inside Excalibur. Hook then asks Emma to kill him to destroy the Darkness, in order to allow him to die as a hero. A sad, crying Emma goes through with his wishes and impales Hook with Excalibur, which kills all of the previous Dark Ones. As Hook dies, his neck wound from Excalibur returns, and Emma is transformed back into her normal self, with Excalibur disintegrating afterwards.
The next day, Belle returns to Gold and forgives him. However, after Emma hears the whispers of the dagger, she meets with Gold, who reveals the truth to Emma, about him using a magic potion to turn Excalibur into a conduit, so that when Emma killed all of the Dark Ones, their magic was transferred into Gold, allowing Gold to reclaim his life as the Dark One, and creating a new Dagger in the process, with a reversed color scheme of a black blade with silver engravings. (Excalibur was still tethered to the Darkness, so when Gold became the Dark One again, the Dagger reappeared.) He adds that he is more powerful than ever before, since he now wields the combined power of every Dark One that ever lived including her. Emma is very cross about this and tells him that she now has leverage over Gold, threatening to tell Belle at any moment the truth about her husband. In exchange for Emma keeping the secret, Gold agrees to help her enter the Underworld to rescue Hook. At the lake, Emma is joined by Gold, Regina, Mary Margaret, David, Robin, and Henry, as they approach the ferry to descend into the Underworld.
