Immediately after becoming the Dark One, Emma disappears and the heroes must band together to save her, but first they have to find her, which will require the help of an unlikely ally. Meanwhile, in the Enchanted Forest, Emma struggles to resist her dark urges as she searches for Merlin in the hope that he can stop her transformation. Along the way to Camelot, she gets help from the plucky and brave warrior princess Merida, as well as King Arthur and his Knights of the Round Table.

In Camelot, as Mary Margaret and David attempt to retrieve the Dark One dagger, Emma uses a Dreamcatcher to look into the past to see how Merlin was transformed into a tree. Together, Emma and Regina figure out the critical ingredient they must acquire to free Merlin, but it's a race against Arthur, who does not want Merlin released. Meanwhile, with encouragement from his moms, Henry musters up the courage to ask Violet on a date. Back in Storybrooke, the heroes break into Emma's house hoping to locate Gold, but what they find will give them a glimpse of Emma's end game. Far from prying eyes, Merida sets about the mission Emma has tasked her with and begins molding Gold into the hero they need to draw Excalibur.

In a Camelot flashback, a chance encounter with Merlin, David, Hook and Belle gives Merida new hope in her quest to save her brothers from the usurping clans of DunBroch. Unwilling to leave anything up to fate, Merida brings Belle along on a dangerous journey that culminates with an invaluable lesson in bravery. In Storybrooke, Regina, Mary Margaret and David discover the spell that would allow one of Merlin’s chosen to communicate with him, but when Arthur fails to reach the missing sorcerer the heroes grow suspicious. Meanwhile, Emma commands Merida to kill Belle in hopes of forcing Mr. Gold’s heroic transformation. With Merida unable to disobey Emma’s orders.

In Camelot, Merlin spearheads a mission to reunite the dagger with Excalibur so he can use the weapon to save Emma from the darkness threatening her soul. With Zelena in tow, Hook, Mary Margaret, David, Regina and Robin steal into Arthur’s castle to take the broken sword from the maniacal king. Meanwhile, Merlin brings Emma along on a journey to confront his ancient nemesis and retrieve the sacred spark he’ll need to reforge Excalibur. Both parties are tested, but one suffers a blow that could derail the entire operation. In a flashback to long before the Age of Arthur, young Merlin finds purpose after he is blessed with magic and immortality, but when he falls in love with a young refugee named Nimue, everything changes for Merlin, as their romance starts a chain of events that touches every one of our present day heroes.

Tensions in Camelot come to a head when Merlin, now under Arthur’s control, delivers an ultimatum to Emma: hand over the Dark One dagger and the Promethean spark or he will kill her entire family. Refusing to give in, Emma and our heroes face off against Arthur, Merlin and Zelena in an epic battle of magic and will. Just when the end is in sight, Emma is forced to make a gut-wrenching choice that no one saw coming. Back in Storybrooke, Zelena’s pregnancy mysteriously accelerates and Hook goes to new and desperate lengths to get answers from the Dark Swan.

In a Camelot flashback, Hook becomes a Dark One and his lust for revenge against Rumplestiltskin is reignited. When Emma and Hook disagree about their best course of action, the final pieces of the puzzle are revealed as events in Camelot catch up to the present and we witness a charged confrontation between the forces of light and dark that sends our heroes on a collision course with destiny. Meanwhile, in Storybrooke, Hook’s centuries old lust for revenge against Gold puts both of their fates in jeopardy while Emma’s love faces the ultimate test as she tries to convince Hook to turn away from the darkness before she, Mary Margaret, David and Regina embark on a new mission as an unsuspected evil is released.