View the Elsa Gallery.
At the end of the show's third season finale, "There's No Place Like Home", Emma Swan (Snow White and Prince Charming's daughter) and Captain Hook accidentally bring back an urn from Rumplestiltskin's vault after their excursion into the past. The urn releases a blue liquid that coalesces into Elsa. She takes off her glove and destroys the urn with an icy blast. She strides out of the barn, leaving a trail of frozen ground. In the fourth-season premiere, "A Tale of Two Sisters," Elsa's story is shown in the present day as well as flashbacks taking place two years after the events of the film. In the past, she discovers that her and Anna's parents set off not on a diplomatic mission, but on a journey to Misthaven—the "Enchanted Forest" where most of Storybrooke's fairy-tale residents came from—to discover more about Elsa's powers, with Anna's travelling to Misthaven to find out more about her parents' voyage. In the present, Elsa is frightened by her sudden exposure into the town of modern-day Storybrooke and re-conjures Marshmallow for protection. Marshmallow is defeated by Regina Mills (the Evil Queen), while Elsa discovers a necklace she gave long ago to Anna in Mr. Gold's (Rumplestiltskin's) shop, leaving her resolved to learn what happened to her sister.
As the season progresses, Elsa befriends series protagonist Emma Swan, Emma and the series' central cast resolving the help Elsa find Anna. The group eventually discovers that a figure named the Snow Queen (a more faithful adaptation of the original fairy tale's character) is somehow responsible for Elsa's trapping in her urn, a woman with similar powers to Elsa. They soon discover that the Snow Queen is Elsa's aunt, Ingrid, who was forgotten from Arendelle when Elsa's mother, Gerda, used the power of the troll king to erase everyones memory of her, not wishing for the world to remember how Ingrid accidentally killed their third sister, Helga. Ingrid soon discovered a prophecy that stated she would one day have the love of her sisters again. With Ingrid's sisters dead however, she believed she would be forced to create new sisters. For this, she chose Emma and Elsa. Ingrid had stalked Emma her entire life in preparation for the prophecy to become true, and manipulated events in Arendelle to bring Elsa to her. Being convinced that she could only gain their love when she was the only one left, she obtained a mirror capable of bringing out the worst in people (the mirror coming directly from The Snow Queen fairy tale). If a piece of the mirror were to get in someones eye, they would see nothing but hatred in the world. Ingrid had planned to use this on everyone in Storybrooke (dubbed the Spell of Shattered Sight), forcing all of its citizens to kill each other while she, Emma, and Elsa remained immune. Ingrid had also used this to her advantage on Anna, forcing Anna to trap Elsa in the urn they originally found Elsa in.
Back in Arendelle, Anna (who has woken up from the spell) tried to track down a device known as a wishing star, which will have the power to bring her and Kristoff to wherever Elsa is. It was apparently the necklace Elsa had given Anna long ago, Elsa using it to bring Anna and Kristoff to Storybrooke. However, the Spell of Shattered Sight had already begun, and the only ones immune to it were Elsa, Emma, and Anna (Anna because it was already used on her a while ago). Tracking down a note written by Elsa and Anna's mother right before her death, the three bring it to Ingrid. It reads that Gerda forgave Ingrid for all that had transpired and still loved her, Ingrid realizing this was what the prophecy meant by regaining her sister's love. Realizing there is no point in killing the town now, the Snow Queen kills herself using the mirror, which deactivates the curse, though a bit of it remained in Storybrooke, reinstating the town line, making it impossible to return to town if you are to leave it.
With the help of Rumplestiltskin, they are able to discover a door that will lead back to Arendelle, though they will not be able to return, as the door will disappear after its use. Anna and Kristoff depart, while Elsa and Emma share a tearful goodbye before returning to Arendelle with her little sister to attend the long-awaited wedding between Anna and Kristoff. In "Poor Unfortunate Soul", it's revealed that, when Blackbeard attacked Arendelle, Elsa shrunk and trapped him and the Jolly Roger inside a bottle, unknowingly trapping Ariel inside as well.
With the introduction of Frozen characters, the season 4 of Once Upon a Time saw a 31-percent increase in ratings from the autumn of 2013 (9.3 million viewers), marking its best ratings in almost two years.