Plot summary
In Oz of the past, Zelena's childhood friend Stanum asks for her help in finding the Crimson Heart, a magical artifact that would be able to restore his human form after having been cursed by the Wicked Witch of the North to slowly turn into tin. Zelena's refusal to give up her magic leads to her betraying Stanum, as she keeps the Heart and leaves him to turn into the Tin Man. In the present, the Black Fairy tempts Zelena with an offer to join her, only to have Zelena oppose her instead.
Regina is brought into the chaos trying to stop her sister and they find the Black Fairy and Gideon in the crystal mines, with the Black Fairy tricking Zelena into utilizing her unstable magic. Realizing that there is nothing left for her in Oz, Zelena decides to acquire the Crimson Heart from there and use it to destroy her magic, undoing the mess she made in the mines. Emma then uses her magic and one of the crystals to bring Mother Superior out of her coma.
Credits
Adapted from Wikipedia contributors, Once Upon a Time season 6. HTML formatting and reference markers removed.