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. (ABC)

In an effort to protect Emma, Regina steps up in a surprising way that will test her mettle as a force for good. King Arthur and Queen Guinevere throw a royal ball to welcome the heroes to Camelot, and back in Storybrooke Hook deploys a tried-and-true technique in hopes of bringing Emma back to the light while the heroes discover that some unexpected baggage has followed them home from Camelot. (ABC)

Determined to help Emma, David embarks on a quest to retrieve a magical relic that Regina could use to communicate with the imprisoned sorcerer, but Arthur insists on joining him because of the dangers ahead. Meanwhile, Mary Margaret discovers the fate of her old friend Lancelot. Back in Storybrooke, Arthur seeks David's help to catch a thief who threatens the safety of the citizens of Camelot. (ABC)

After receiving a cryptic warning from Lancelot about Arthur's intentions, Mary Margaret realizes Arthur may be the heroes' biggest threat, but when she is unable to convince David of the danger, she takes matters into her own hands. Meanwhile, Hook's unwavering love for Emma provides a glimmer of hope in her struggle against the unrelenting voice of Rumplestiltskin. (ABC)

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. Back in Storybrooke, the heroes break into Emma's house hoping to locate Gold. (ABC)

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. In Storybrooke, Regina, Mary Margaret and David discover the spell that would allow one of Merlin's chosen to communicate with him. (ABC)

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. 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. (ABC)

Tensions in Camelot come to a head when Merlin, now under Arthur's control, delivers an ultimatum to Emma. Refusing to give in, Emma and our heroes face off against Arthur, Merlin and Zelena in an epic battle of magic and will. Back in Storybrooke, Zelena's pregnancy mysteriously accelerates and Hook goes to new and desperate lengths to get answers from the Dark Swan. (ABC)

Zelena and Arthur journey to DunBroch on a mission to retrieve an enchanted relic that will provide the advantage they need to vanquish Emma and our heroes. Their path will cross with Merida's, who is on a journey of her own to pay a debt that her father, King Fergus, owed to the Witch when he died. In flashbacks, Merida learns about bravery and honor as she trains for combat and rides alongside Fergus into the infamous battle that claims his life. (ABC)

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. 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. (ABC)

The dark siege of Storybrooke is underway as all of the resurrected Dark Ones target a living soul for sacrifice so that they can return to the realm of the living. Emma, however, refuses to give up, knowing that she is responsible for righting all the poor choices she made as Dark One. In a Fairy Tale Land flashback, the Evil Queen tests Hook's mettle by forcing him to confront a demon from his past before contracting him to dispatch one from hers. (ABC)

Emma, Regina, Mary Margaret, David, Robin, Henry and Gold arrive in the Underworld, on a mission to rescue Hook, only to discover that their search will be more difficult than they had anticipated. Meanwhile, in an Enchanted Forest flashback, a familiar face from the Evil Queen's past returns to present Regina with the perfect birthday gift. (ABC)

An escapee from Hades' prison informs the heroes that Hook is being held captive. But before the heroes can rescue Hook, they must face a terrifying beast that guards the prison. Meanwhile, in an Enchanted Forest flashback, a young Snow White struggles to preserve peace in her kingdom and must learn how to be a true hero if she wants to one day become queen. (ABC)

Hook's captivity takes a dark turn when Hades threatens to condemn him to the River of Lost Souls. Meanwhile, Gold is suddenly eager to help Emma, and Snow, David, Regina, Robin and Henry find Hook so he can return home to Belle. In Fairy Tale Land flashbacks, Rumple and Milah face a life-or-death crisis concerning their son, Baelfire, that forces Rumple to make a deal that will haunt him. (ABC)

Finally reunited, Hook and Emma must find a way to defeat Hades so that they can return to Storybrooke. A surprise visit from Hook's departed brother Liam just might provide the lead they were looking for -- a book that contained the key to his downfall. Tired of watching from the sidelines, Henry secretly takes matters into his own hands while the others search for the book. Meanwhile, Hades redoubles his efforts to trap the heroes in the Underworld by playing them against one another. (ABC)

At the mercy of Hades, Gold creates a portal to Storybrooke, which transports Belle, Zelena, and Baby Hood to the Underworld. While Zelena and Regina get into a sibling squabble, Belle is reunited with Rumple only to learn some tremendously shocking news that will change her life forever. Meanwhile, Snow and David hopelessly attempt to send a message from the Underworld to their son, Neal. In flashbacks, Hades learns about Zelena's very special spell and pays her a visit in Oz with the intention of striking an alliance. (ABC)

Belle turns to Rumplestiltskin with hope of finding a way to protect their child from Hades. Meanwhile, Hades secretly meets with Gaston in the Underworld and encourages him to seek revenge on Rumplestiltskin. As the heroes try to find a way to defeat Hades, Emma has a terrifying nightmare that she realizes is beginning to come true. And in flashbacks, Belle and Gaston meet for the first time, but their blossoming romance comes to a halt on the eve of the Ogre wars. (ABC)

In flashbacks, Ruby and Mulan find themselves in Oz, where they meet Dorothy. After the three witness Zelena's return to Oz, they look for a way to defeat her once and for all. However, Dorothy mysteriously disappears, and Ruby's search for her new friend lands her in the Underworld. Reunited with the heroes, Ruby teams up with Emma, Regina and Snow to continue looking for Dorothy. (ABC)

Now that Hades and Zelena are reunited, Hades tells Zelena that he wants a future with her outside of the Underworld. All she needs to do is heal his heart with True Love's Kiss so they can leave the Underworld and trap the heroes there for eternity. When Regina overhears of this plan, she enlists Cora's help to find a way to separate Zelena from Hades. Meanwhile, David finally meets James, only to realize that his twin is determined to seek revenge on him for stealing the life he could have had. (ABC)

Hades turns to the heroes to ask for help with getting Zelena back from Rumplestiltskin and Peter Pan. In return, he offers to take all their names off their tombstones. However, when Hook is still unable to leave, he and Emma must journey into the depths of the Underworld. Meanwhile, Cruella De Vil is determined to keep the heroes trapped in the Underworld. (ABC)

Emma, David, Regina, Robin and Henry are finally back home in Storybrooke and reunited with Snow, but, unfortunately, they still have to contend with Hades, who continues to deceive Zelena as he lays out his plan to use the all-powerful Olympian Crystal to take over the town. The heroes desperately search for a way to defeat Hades while Hook does the same in the Underworld, looking for those missing storybook pages. (ABC)

When the heroes discover Gold has stolen Hades' Olympian Crystal and tethered all of Storybrooke's magic to it, they set out to stop him. Henry decides he no longer can stand all the pain magic has caused his family, so he goes rogue, with Violet in tow, to destroy magic once and for all. Meanwhile, Zelena, Snow, David and Hook attempt to open a portal that will return Merida and the other Storybrooke guests to their homes. (ABC)

With the possibility of magic being destroyed and the fate of Storybrooke hanging in the balance, it's a race for Emma and Regina to track down Henry before Gold can find him first. Elsewhere, Snow, David, Hook and Zelena are imprisoned and must contend with very two very disturbed individuals that may give Gold a run for his money. (ABC)