One of the principal legends about Idunn concerns an episode in which she is kidnapped by the giant Thiassi. The story of Idunn's kidnapping and recovery is recounted in the "Skaldskaparmal" section of the 'Prose (or Younger) Edda'. On a journey through the mountains, the gods Odin, Loki, and Hoenir (Vili) grew hungry. They came down into a valley and saw a herd of oxen, so they took one of the oxen and set it in an earth oven. Several times, judging that the meat should be done, they checked the oven, only to find that it was still raw. A large eagle, sitting in an oak tree above them, said that he was responsible, and that if the gods would grant him his fill of the ox, the oven would cook it. The gods agreed. The eagle sat on the oven and immediately devoured the ox's two hams and both shoulders. Loki became angry, snatched up a great pole, and swung it at the eagle with all his strength. The eagle jerked away and flew up with one end of the pole stuck to his body and the other end in Loki's hands. The eagle flew so that Loki's feet banged down against the stones and gravel and trees and Loki thought that his arms would to be wrenched from his shoulders. He shouted and begged the eagle to release him, but the eagle said that Loki would never get free unless he solemnly vowed to lure the goddess Idunn to come outside of Asgard with her apples.
Loki accepted the terms. The eagle released him and he found his way back to the other two gods, but he told them nothing about what had transpired. At the agreed time Loki lured Idunn out of Asgard into a forest, saying that he had found some apples that he thought she would want, and told her to bring her apples along to compare them. Then the eagle arrived. The eagle was really the giant Thiassi in disguise. He snatched up Idunn and flew away with her to his home, called Thrymheim.
Without Idunn's apples of youth (See Idun's Apples), the other gods soon became grey and old. The gods then held a council concerning her mysterious disappearance and asked each other when they had last seen Idunn. They discovered that the last time she had been seen was with Loki. Loki was arrested, brought to the council, and threatened with death or torture. In terror for his life, he said he would go in search of Idunn in Jotunheim if Freya would lend him the shape of a falcon she possessed. In this falcon shape, he flew north to Jotunheim and arrived one day at Thrymheim. Thiassi was out at sea in a boat, but Idunn was at home alone. Loki found her and turned her into the form of a nut. He held her in his claws and flew as fast as he could back toward Asgard. When Thiassi got home and found Idunn gone, he assumed his eagle shape and flew after Loki. He flew so fast and hard that he caused storm winds. From Asgard, the gods could see the falcon approaching with the nut in its claw, and the huge eagle chasing it. The eagle was catching up. They went outside their fortification and piled up loads of wood shavings. As soon as the falcon flew in over the wall to safety, the gods set fire to the wood shavings. Unable to slow down quickly enough, the eagle flew into the fire, and the gods were able to kill it within Asgard's gates. The killing of the giant Thiassi was a deed of great renown among the gods, and as soon as they had Idunn's apples again, they regained their youth and vigor.