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.