The episode's flashbacks deal with the heroes final days in Camelot and how they returned while the present storyline focuses on the consequences of Hook as the Dark One.
The Jolly Roger is featured in the forest.
In the Dark One's Vault, the Darkness enters Hook, making him remember all of the darkest moments of his life, causing him to scream in agony, before reforming him outside of the Vault as the newest Dark One. Like Emma, he is greeted by a manifestation of Rumplestiltskin, who offers Hook a chance to seek revenge on the living Rumplestiltskin, but Hook is reluctant to take up the challenge, knowing what the manifestation is, and not wanting to give in to the darkness.
David, Mary Margaret, Merlin, and the others search for Hook near the Vault of the Dark One, but they are too late, as Hook is no longer there. David expresses his frustration at being thwarted when they were so close to wiping out the Darkness, but Mary Margaret reminds him that they would have chosen to save their loved ones in any situation, which Emma did. Merlin tells them that this was the darkest possible future he saw for Emma, and that now, their fight to destroy the Darkness had become much more difficult. Merlin sends Sir Lancelot to find his mother, the Lady of the Lake, as she can help them defeat the Darkness. Merlin heads back to the diner while the others split up to continue their search for Emma and Hook.
When Hook says that the manifestation is lying, that there's no way to go back to Storybrooke, the manifestation taunts him by saying that he could use the Dark Curse to return. When Hook says that he would never use Emma's heart to enact the Curse, the manifestation says that for Dark Ones, "there is always a loophole." Suddenly, Emma appears as the other Dark One, to convince Hook that they can work together to stop the Darkness by bonding together in love, and their kiss causes the manifestation of Rumplestiltskin to disappear. Later that night, Hook feels the pull of Excalibur despite Emma's claim that it disappeared. When he calls Emma back, she admits that she has Excalibur, because Hook told her that he couldn't resist the darkness, making Hook furious with her lack of trust in him. Hook tells her that he had always trusted her, and that he was the one who told Emma's parents that she had to decide on her own not to crush Merida's heart in the past, before disappearing. Later on, Emma is forced to summon him with the Excalibur fragment later, just to have a conversation, angering him even more by the fact that she had used Excalibur to "force" him to come. She gives him Excalibur so that he can be in control of himself and they have a kiss.
It is soon revealed that just seconds after Merlin made the message that would be later be received in Storybrooke, Hook appears to ask for a spell that will allow them to return to Storybrooke and seek revenge on Gold. Merlin says that this is impossible, since the only spell that could do this was the Dark Curse, but he would have to crush the heart of "the thing they love most" to enact it, and Hook would never kill Emma. Hook then takes Merlin’s heart, and the manifestation of Rumplestiltskin morphs into one of Nimue, who appears to see Hook crush the heart. Emma arrives, devastated to find that Hook has tricked her. When Emma sees Nimue, she is confused, and Nimue explains that when Hook crushes Merlin's heart, it also counts as her crushing his heart, since she is "in all the Dark Ones," which will allow Hook's Curse to work. Emma tries desperately to convince Hook to stop, telling him that he doesn't need revenge, and that his happy ending was with Emma. She warns Hook that if he crushes Merlin's heart, he will destroy his happy ending. However, Hook states that it was "puppy-eyed" "Killian Jones" who had fallen in love with her, claiming that that his former self died when she "turned him into a Dark One." Then, he suddenly crushes Merlin’s heart in his hand, to enact the Curse. Then, Emma tells Merlin that she was sorry, before dissolving his body and telling him to rest in peace. Unable to stop the Curse, a heartbroken Emma knocks Hook out, before using a dreamcatcher to erase the memories of Hook, to prevent him from remembering why he cast the Curse. Then, at the behest of the manifestation of Rumplestiltskin, Emma brings her parents, Henry, Regina, Robin, and Zelena (whom she reinstalled the magic-inhibiting cuff on) back to Granny's Diner in an unconscious state, before using the dreamcatcher to erase all of their memories, and to add a forgetting component to the Curse, to keep Hook from remembering that he was ever a Dark One. Meanwhile, Excalibur's fragment is returned to the Stone, much to the shock of Arthur, and Guinevere points out the cloud of the Curse, which Arthur states was the darkest of magic. Then, Merida, Arthur, and Guinevere look on as the purple cloud of the Dark Curse engulfs Camelot.
At Emma's home, Hook, now endowed with the Dark One's powers, starts to embrace his new life in darkness, as Zelena watches in delight in front of a immobile Emma. Upset over what she did to him, Hook takes the dreamcatcher to erase Emma's memories of the events in Storybrooke, while Zelena places the anti-magic cuffs on her. Mary Margaret, David, Henry and Regina arrived to see Emma powerless. She tells them about Hook, and they reason that they can simply use the dreamcatchers to regain their memories, and find out what Hook’s plans are, but Hook had taken all of the dreamcatchers. At the pawn shop, Hook shows up to challenge Gold to a battle on the Jolly Roger, so that he can finally get his revenge, giving Gold a cutlass to use for the battle, while Hook wields Excalibur. As Gold and Belle alert the others about Hook's challenge, they debate about whether they should leave the cuffs on Emma, and with objections from Henry, they agree to leave her at the house with the cuffs, as they devise a plan to stop Hook.
Zelena heads to the hospital to find her child but finds only Regina, telling her that the child is Robin's and that her insanity has to end. However, she allows Zelena to meet with Robin and the baby back at their home. Vowing that they will never be a family, Robin tells Zelena he will allow her to visit the child, as long either he or Regina are present. Zelena then has her first real bonding moment with her new baby.
Regina asks Merida to keep a eye on Emma, only to have Hook appear in front of the archer and knock her out. When Emma asks Hook what he plans on doing, he replies by saying that since he was now the Dark One, he was a "free man" and now sought revenge on Mr. Gold. He says that he is no longer bound by love or by Emma as "a distraction" from his true goal. He goes on to tell her that she didn't need any villains to isolate her, because she was already good at doing that to herself. He says that each time Emma is in trouble, she pushes away all of her loved ones, including Hook, because she thinks that she can solve all of her problems on her own, and he tells her that her personality will keep her as an "orphan" forever. When Emma asks Hook why he's telling her that, Hook says that he's doing it because he wants to hurt her like how she hurt him, before vanishing. However, Hook's appearance had given Emma enough time to escape, to warn Gold and ask him to succeed in distracting Hook during their upcoming duel, so that she can retrieve the dreamcatchers.
Emma then teams up with Henry to form "Operation Cobra: Part II." Henry offers to create a locator spell, which leads them to a seemingly defenseless building. On board The Jolly Roger, Hook and Gold start their battle, with Hook using magic to repair Gold's crippled leg to give him "a fair fight." This gives Henry and Emma enough time to find the dreamcatchers, only to discover that they are protected by a spell. Meanwhile, Hook backs Gold into a corner and draws blood, saying that he will enjoy the final blow. Henry decides to trust Emma again, and uses a magic potion to remove her cuffs, which allows her to remove the dreamcatchers from Hook's protection spell. Back on the ship, Gold is able to take control of the fight, when the manifestation of Rumplestiltskin distracts Hook on how he can kill the action Mr. Gold, who seizes the opportunity to drop a weight on Hook and seize Excalibur. When Hook awakens, Gold decides to let Hook live forever, with the memory of being beaten by Gold. Hook says that that his victory was only temporary, and disappears. When Gold meets Belle at the well, she informs Gold she wants to "step back" in their relationship. She tells him that she spent too many years trying to mend Gold's heart and now, she wants to protect her own heart, leaving a heartbroken Rumplestiltskin at the well. As Emma and Henry distribute the dreamcatchers to everyone and restores their memories, Emma has a realization as she learns that Hook intentionally lost the duel to Gold, so that he could get a drop of Gold's blood. It turns out that Hook planned to use the blood at the pond, for a spell that opens a portal into the Underworld. Hook opens the portal and summons all of the previous Dark Ones (minus Rumplestiltskin), led by Nimue, to aid him in destroying light magic.