The people of Storybrooke are forced to turn to Zelena for help when they realize that Emma has disappeared from the real world. Meanwhile, Emma tries to fight the temptations of the Darkness and meets a possible ally.
At Camelot six weeks earlier, Regina tries to impersonate the Savior when Arthur explains how they must free Merlin and fulfill the prophecy. However, one of the knights knows Regina as the Evil Queen and swears revenge. In the present, Hook tries to free Emma from the Darkness, while a Fury comes to take Robin to the Underworld as the price for the magic Regina did in Camelot.
Arthur turns to David for help when he discovers that someone has stolen a magic bean from the Reliquary, and they turn to Belle for help. Meanwhile, Emma invites Hook to have lunch with her as part of her plan to free Excalibur from the Stone. In Camelot, David and Arthur quest for a toadstool that may provide the key to freeing Merlin.
In the past, Arthur threatens to tear his marriage asunder in his quest to restore Excalibur. Meanwhile, Guinevere and Lancelot enter the Dark One's Vault to find the Dagger, and Guinevere is forced to make a deal with Rumplestiltskin. Six weeks earlier in Camelot, Mary Margaret and Lancelot prepare to hide the blade, but discover that Arthur has an unexpected ally.
Emma discovers how Merlin was imprisoned in a tree, and must take drastic steps to acquire the one necessary ingredient... steps that will lead her into darkness. Meanwhile, in the current Storybrooke, the townspeople decide to have a block party to lift everyone's hopes. Henry turns to Emma for help find Violet's missing horse, and Regina and the others learn that Emma has Excalibur.
Six weeks earlier, Belle travels with Merida to help her defeat the clans and rescue her brothers. In the present, Emma continues her efforts to turn Gold into a hero by forcing Merida to threaten Belle's life. When the couple try to leave Storybrooke, Merida transforms into a bear and attacks Belle. Meanwhile, the others discover that Arthur is lying to them.
Six weeks earlier, Emma and Merlin quest for the Flame which can reunite the two parts of Excalibur. Meanwhile, Regina and the others sneak into Camelot to recover the two pieces. However, things go horribly awry. In the distant past, the origin of Merlin's powers and the first Dark One are revealed.
In the present, Hook takes desperate measures to find out what Emma is hiding from him during their time in Camelot. Three weeks earlier, Emma must make an impossible choice when she must decide whether to give up the Darkness or use it to save her future.
Arthur convinces Zelena to help him seek out a magical helm that will let him compel his men to find Merlin and Emma. However, they soon cross paths with Merida, who is seeking the helm because her father Fergus bargained with the witch to obtain it. Together with two new friends--Ruby and Mulan--Merida races to find the helm and the man who killed her father.
Three weeks earlier, Hook sets in motion the events that will lead to everyone returning to Storybrooke with their memories erased. In the present, Hook challenges Gold to a duel but has an ulterior motive. Meanwhile, Emma tries to convince Henry that he can trust her even though she's a Dark One.
The Dark Ones bind themselves to the living so that they can remain in the real world... and the townspeople will be taken to the Underworld in their place. Emma tries desperately to find a way to stop the Dark Ones' plan, but the others accept the inevitably and spend their last moments together. In the past, Regina tests Hook by leading him to his father and offering him transportation to the real world once Hook proves that he has no weaknesses for Cora to exploit.
After a meeting with a former friend, Emma arrives in the Underworld with the others. They find a "dark" Storybrooke ruled over by Cora, and Cora threatens the soul of Regina's father if Regina doesn't return to the land of the living. In the past, Cora finds the perfect present for her daughter's birthday.
In the present, Hook helps his cellmate to escape so that she can tell his rescuers where he is. Mary Margaret meets an old friend, and must give him the courage that he once gave her. In the past, a young Snow faces her first challenge when bandits threaten her people.
Gold offers to help Emma find Hook, but needs Milah's help to breach the gates of the Underworld. Meanwhile, Regina consults Cruella to find out what happened to Daniel. In the past, Rumple must make a deal to save the life of his son after Bae is bitten by a snake.
In the Underworld, Hook is reunited with his older brother Liam. Liam offers to help the group find the Storybook and the pages containing Hades' weakness... and then makes a deal with the Lord of the Underworld. In the past, Liam leads a mutiny against Captain Silver, and makes a deal with Hades to ensure his success.
Zelena, Belle, and Zelena's baby daughter are all pulled to the Underworld by Gold's portal spell. Meanwhile, in the past, the connection between Hades and Zelena is revealed.
In the Underworld, Hades convinces Gaston to hunt down the Beast that killed him--Gold. While Belle tries to find an alternative, Hades offers her a deal of her own. In the past, the circumstances behind Gaston and Belle meeting for the first time are explored.
Ruby tells the others how she arrived in Storybrooke after traveling to Oz with Mulan and meeting Dorothy. Meanwhile, Mary Margaret and David make plans for one of them to go back and be with their son Neal, and Belle comes up with a way to protect her and her unborn child from Hades.
Regina realizes that Zelena plans to try and reform Hades, and turns to Cora for help when she refuses to accept that her sister can turn the villain from evil. Meanwhile, James abducts David and takes his place. In the past, Regina and Zelena are briefly united as sisters but tragedy ensues.
Hades asks the heroes to help him make sure that Gold and Pan go through with their end of the deal so that he can get Zelena back. Once they do, the Underworld begins to fall apart and Henry offers to help the other residents find their unfinished business. Meanwhile, Cruella and the Blind Witch take steps to make sure the heroes don't escape. In the past, Emma meets a sympathetic bounty hunter who helps her find out about the parents who abandoned her.
The heroes return to Storybrooke but discovers that Hades has plans of his own to conquer the town. In the Underworld, Hook finds a new ally and together they set out to find the Storybook pages that show Hades' weakness.
Henry sets out to destroy magic and takes Violet with him. While Emma and Regina go after their son, the others try to create a portal to the Enchanted Forest and are transported to a mysterious land ruled over by a violent Warden with a grudge against Gold.
Henry manages to destroy magic, trapping half of his family in the Land of Untold Stories.To restore the magic, he must convince the people of New York to believe in magic. Meanwhile, Regina faces her darker self, while Gold makes a deal with the mysterious Warden.