Brock rolled out as Anna was walking through the forest, stopping her and introducing himself as "Brock the Mystical". When Anna tried to leave, the troll revealed that he knew of her missing memories, thus convincing the princess to accompany him to his hut.
At his hut, Brock went on to tell Anna about the night that she was hurt by Elsa and how her family took her to the Valley of the Living Rock to save her. Informing Anna that Pabbie had removed her memories of magic to keep her safe, Brock offered to restore them. Stunned, Anna said she had to leave, though Brock told her that the offer remained open should her mind change.
After consideration, Anna returned to Brock's hut with Elsa and Kristoff so that the trolls could recover her memories. Brock excitedly waved them into his hut, and after becoming acquainted, the troll set the trio the task of finding special plants for the potion he would make to restore Anna's memories. After the three returned with the ingredients, Brock began to mix the potion. When he finished, Brock handed Anna a smelly brown drink, referring to it as a "memory draught". After the princess drank it, Brock circled the cauldron three times while chanting in a mysterious language. He told Anna to also circle the pot, except while hopping on one foot and then clucking like a chicken. After Brock told her to stop, the hut fell silent, save for Kristoff's snickering.
To test the success of the potion, Brock asked Anna what color the sky was, what color her hair was, and what her favorite food was. After Anna responded correctly to all three questions, the troll announced that the potion had been a success, though the frustrated princess informed him that the questions had nothing to do with Elsa's magic. After some thought, Brock suggested more clucking. Elsa instead proposed that her sister try remembering something about the two of them. When Anna shared the story of the time she and Elsa went ice fishing, Brock exclaimed at how wonderful the story was.
Thinking that his magic had worked, Brock went to hug the trio but knocked a burning coal from the fire, causing a book to be set alight. Before Kristoff could stand on the book, Brock threw it outside, where it landed in a patch of dry bushes, causing the fire to spread. Brock panicked after some burning leaves fell onto his hut, prompting him to try and blow out the flames. Kristoff, Elsa, and Anna proceeded to knock over the hut and stamp out the flames; however, the fire continued to spread. Realizing that they could not control the fire alone, Anna left to recruit the aid of the other trolls, though their combined attempts still proved futile. Rising on a pillar of ice over the trees, Elsa summoned snow, which fell upon the flames and doused them.
With the crisis averted, the trolls cheered and invited Brock to live with them while his hut was rebuilt. Though Brock left certain that his potion had managed to restore Anna's memory, it had in fact failed.
Brock is a friendly troll who is ready to offer his expertise to help others. He even regarded his books as friends, such that he reprimanded Kristoff for sitting on them. He is conscious about others, having moved away from the other trolls since his attempts at mystical powers always caused accidents. But despite this geniality, Brock is somewhat eccentric, to the point that Anna regarded him as "kooky".
Brock is exceptional from the other trolls due to his ability to move in daylight, during which others of his kind remain in stone forms.
To conjure magic, Brock uses potions and spells; he believes that he will be as great as Pabbie one day.
- The other trolls refer to Brock as "Brock the Rock" because they feel he is as smart as a rock.