Regina poses as the Savior to convince Arthur to help find Merlin.
In the Character's Past
In the early years before Camelot, we meet the stable boy who would later become the once and future King, Arthur, who was motivated to fulfill his prophecy of finding Excalibur at a very early age, which he would follow through as a adult years later (five years before the Storybrooke residents' arrival), when he pulled the sword out the stone. While he proclaimed to his kingdom that he had found the entire sword, Arthur, like Merlin, only spoke in half truths, as the King left out the part about the sword being split in half after he shows a portion from the case. He even tells Guinevere that he has studied the translations of the Camarthen Scroll, which contain the three elements: the star, the eye, and the sun, but can't decipher the clues. Arthur's quest to reunite Excalibur has lead to his long running obsession to seek out the missing sword piece, which in turn leads to shutting Guinevere out of his life, that of course would lead to her turning to Lancelot, who would plan a party for her and made her feel loved, and would stay behind to watch Guinevere as Arthur began his search for the missing dagger.
With Arthur out on his mission, Lancelot and Guinevere decided to seek out the dagger themselves, by using Merlin's gauntlet that lead the two to a magical passageway that was located in the well. However, the passageway was also a deadly trap as the Dark One's spirits emerge and almost killed Lancelot, but he eventually survived. This would lead to a kiss between Lancelot and Guinevere. The two would walk through the doorway and discover the dagger, but couldn't grab it thanks to a protection spell placed on it by Rumplestiltskin, who in exchange for the gauntlet, offered the magical sands of Avalon, which can be used to solve any problem at their choosing. Unfortunately, and despite Lancelot's warning not to accept it, Guinevere decided to take the deal and went back to Arthur. However, when she returned to Camelot, she was confronted by Arthur, who knew about her and Lancelot, and threw the enchanted sand of Avalon on her, making his wife obedient with whatever he wanted, and used more of the sand to create his Camelot empire.
In the current universe of Camelot, King Arthur showed David the broken Excalibur after reading books on how to make it whole, and when he asked David to help him find the other half, David realized where the portion of the dagger came from. As for the dagger itself Swan is being drawn to it with each passing day, but is repelled by a protection spell put in place by Regina. The manifestation of Rumplestiltskin continues to consume Emma's mind to the point of Emma nearly hurting Hook. The effects are starting a toll on Emma, prompting a intervention from Hook, Henry, Mary Margaret, David, and Regina, but before they can come up with a solution, Mary Margaret asks David for a moment alone to talk to her husband, and as expected, was to tell him about King Arthur. However when she told him about Lancelot being alive and the message he gave her, David ended up defending Arthur, believing that he is being very trustworthy. Mary Margaret on the other hand, believes David has become friends with Arthur because he misses the life he enjoyed back in the Enchanted Forest and that fulfilling it as a Knight in the Round Table will make him important again.
David told Arthur they were going to restore Excalibur right this second and revealed that Emma was the Dark One and that Lancelot was back. David believed that he had brought the dagger to hand over, but Mary Margaret brought it to Lancelot to hide, who then took her to the place that he and Guinevere had discovered. Mary Margaret realized that she has been here before in a vision where Emma pulled her heart out, but before they can carry out their plan a deranged Arthur arrived and pulled a sword on them and demanded Mary Margaret hand over the dagger. Arthur then used the dagger to command Emma to come to him, only to be fooled by Mary Margaret as the dagger she gave him was a fake, and David was on to this plan all along. The threesome took Arthur to Granny's Diner, where Guinevere arrived and uncuffed him. Aware that Guinevere was still under a spell, she broke Lancelot's heart by telling him that she loved Arthur. Arthur by the way, also had a plan up his armored sleeves, as Guinevere blow a kiss out the sands on David and Mary Margaret, then tossed Lancelot in jail, where he was joined by another cellmate, Merida. With the Charmings now under Arthur's powers, they went to Regina and deceive her, with Mary Margaret saying "Making Excalibur whole is our best chance to help Emma."
Meanwhile, Henry brought Emma & Hook to the stables, but he really just wanted to see Violet, even though he didn't tell her about his birth mother being the Dark One. Hook made Emma forget all about Rumple with a romantic horseback riding. She needed to trust someone, and afterwards the two kissed in a field of roses.
In Storybrooke, Emma still had a pink rose called middlemist from Hook. As for Gold however, he was still locked up saying he is not ready to be a hero. Emma wasn't convinced, telling Gold that she could turn him into one with a little time. It also turned out that Emma has another person working for her as her minion, which is revealed to be Merida, and she has her heart. Emma wants the archer to carry out her plan, telling Merida "I need you to make him brave."