As an 8-year old orphan, Kristoff is first seen attending a session of ice harvesting amongst a group of burly harvesters. Accompanied and assisted by Sven, Kristoff watches intently as the elders go about their work for the day. By night, Kristoff manages to obtain his own, small block of ice, much to his pride, and follows behind the harvesters as they make their leave from the mountains.
Meanwhile, an accident has occurred at the castle of Arendelle. Princess Elsa (born with the power to make ice and snow) has accidentally struck her sister, Anna, in the head with her magic. Unconscious, Anna is taken into the care of her parents, King Agnarr and Queen Iduna, and rushed to a legendary valley populated by a colony of trolls. During their trip, the horses of Agnarr and Iduna rush passed Kristoff, while leaving a peculiar trail of ice. Curious, Kristoff and Sven follow the royal family, arrive in the valley, and watch in hiding as their business unfolds. While covered, they are discovered by a female troll named Bulda. Bulda instantly takes a liking to Kristoff and Sven and decides to adopt them.
In the following years, Kristoff is raised under the loving and supportive care of the trolls. Thirteen years after his adoption, Kristoff has since become a professional ice harvester, seen selling ice alongside Sven in Arendelle's village square.
On the night of princess Elsa's coronation as queen, her powers over ice and snow are revealed to the public. Stunned by the revelation, the people call Elsa out as a "monster", prompting her to leave the kingdom. During her escape, Elsa accidentally plunges Arendelle and its surrounding territory into an eternal winter. She retreats to the North Mountain, and area Kristoff is near. As the storm covers the land, Kristoff and Sven travel to Wandering Oaken's Trading Post and Sauna for supplies and carrots. Inside, he encounters an adult Princess Anna, who is in search for Elsa. When asked by the shop's owner, Oaken, about the peculiar weather, Kristoff nonchalantly notes it to be coming from the North Mountain. Enticed, Anna attempts to gain further information from the mountain man, but during the interaction between him and Oaken, the prices for Kristoff's mountain gear are revealed to have been piked, due to the supply and demand issue stemmed from the unexpected winter. Kristoff makes the poor choice of calling Oaken a "crook" in response and is thrown out of the establishment immediately. Despite his lack of goods, Kristoff and Sven retire to a nearby barn to rest of the night. After a lullaby to welcome sleep, the duo is visited by Anna. The princess asks Kristoff to escort her to the North Mountain, though Kristoff refuses. Anna then reveals to have purchased the material Kristoff wanted from Oaken and demands that the mountain man complies with her request in gratitude for her services, additionally claiming to know the secret to stopping the winter. With his business in peril, Kristoff assent to Anna's proposal, and he, Sven, and the princess set off that night, per the latter's orders.
As the travel begins, Kristoff inquires Elsa's reasoning behind freezing the entire kingdom. He's incredulous at Anna falling for Hans and wanting to marry him within hours of meeting him. The conversation is cut short when a pack of hungry wolves attacks them. They manage to evade the wolves by jumping over a chasm, but the sled is lost, plummeting to the bottom of the cliff, along with the supplies. After taking a few moments to recover from the scare, and after seeing the trouble she's caused, Anna offers recovery for the lost sled and supplies, in addition to continuing her search alone, in hopes of saving Kristoff and Sven from any further turmoil. As she prepares to leave, Sven urges Kristoff to continue aiding her, though the mountain man expresses apprehensions in fear of further danger. However, knowing Anna would die on her travels should she endure them alone, Kristoff makes the decision to continue forth, by her side, to help aid and protect the princess throughout the remainder of the journey, much to Sven's delight.
The following day, the group comes across a talking snowman named Olaf, who dreams of experiencing summer. Knowing the consequences of this dreams, Kristoff sadistically shows the desire to reveal exactly what happens to snow during the summer season, but a protective Anna prevents him from doing so. Olaf's dream aside, the snowman also knows the location of the Snow Queen and offers himself as the group's guide, to which Anna and Kristoff accept. After a bit of travel, the group finally arrives at the Ice Palace, leaving Kristoff in awe at the beauty ice is capable of forming. In spite of this, Kristoff is forced to remain outside per Anna's wishes, as she wouldn't want Elsa to cause more havoc at the sight of a man by the former's side (referring to the debacle with Hans). Kristoff expressed grief over the matter, but eventually abides and waits patiently outside alongside Olaf. Kristoff enters the ice palace upon hearing a commotion, which appears to have resulted in Anna being struck in the heart. Kristoff remains by her side as Elsa orders the group to leave, which Kristoff tries to oblige to out of fear, though Anna refuses to leave the palace without her sister.
This forces Elsa to create a giant snow monster named Marshmallow to forcefully throw the group out. Kristoff simply accepts Marshmallow's rudeness, but Anna retaliates by throwing a snowball at the beast, which immediately angers him. Kristoff and Anna rush down the mountain while being hotly pursued by Marshmallow. Throughout the chase, Kristoff continuously goes out of his way to protect Anna, proving his selflessness, and is evidently highly impressed by her self-sufficiency against the snow beast. Once they reach the cliff of a mountain, the ice harvester creates a snow anchor for the two to safely travel down, though Marshmallow grabs hold of the rope and slowly pulls them to their doom. Kristoff tries to free Anna but is briefly knocked unconscious in the process, leaving Anna to save them both by cutting the rope with Kristoff's knife.
The two land safely on a blanket of snow below, and are reunited by Olaf and Sven. After Kristoff assists Anna in getting back onto her feet, the two share a loving glance in mutual gratitude for their previous actions. Unfortunately, this is short lived, as Anna starts to panic over Elsa's refusal to cooperate, with the storm still going strong. She couples her anxiety with the fact that the blizzard will result in the failure of Kristoff's ice business, to which the latter replies by calmly assuring his ice business is no longer a priority. Just then, Kristoff notices Anna's hair beginning to turn white and deduces that the phenomenon is due to Elsa striking Anna in the heart. Fortunately, Kristoff believes he has a solution, and leads the gang to the Valley of the Living Rock, where the trolls reside.
Along the way, Kristoff's appears to have abolished his defenses and starts to open up to Anna by revealing his past, including how he and Sven were lonely orphans until they were taken in by their adoptive family, which Anna is touched by. Though he loves his family, Kristoff warns Anna that they can be rather overbearing, stubborn, and loud at times, to which Anna responds by claiming they sound wonderful. With his insecurities somewhat lowered, Kristoff introduces Anna and Olaf to his family, who are currently dormant as rocks. Anna and Olaf assume Kristoff is mentally unstable, and make an attempt to leave until the rocks finally reveal themselves to be sentient trolls. A warm reunion follows, as Grand Pabbie, who Kristoff is searching for to help Anna, is said to be asleep. The trolls then notice Anna standing by, and Bulda immediately assumes her to be Kristoff's girlfriend. The two deny any romance, but Bulda and the trolls try to convince them otherwise, telling them they're meant for each other. They arrange an entire wedding, but before it is complete, Anna nearly faints and loses balance, with Kristoff catching her in his arms and anxiously informing the trolls she's freezing cold. Pabbie, although he is powerful, is unable to heal her, and he tells them that unless Anna can perform an act of true love, she'll freeze and die. Bulda believes the act of true love would perhaps be "true love's kiss," which prompts Kristoff to take Anna back to Arendelle so she may receive a kiss from Hans, her supposed true love.
Once they enter the kingdom and the castle gates, Kristoff escorts Anna to the care of the royal staff. Now that she has returned to her royal care, as well as her true love, Kristoff feels his time of usefulness to Anna has reached its limit, and sadly takes his leave from Arendelle with Sven by his side. Unbeknownst to him, Hans doesn't really love Anna, as he was only using her to become the king of Arendelle, thus denying her a kiss and leaving her to die in a locked room within the castle. Afterward, Olaf, who got separated from the group during their arrival in the city, finds Anna lying in the library freezing, and starts a fire to lessen her curse until an alternative plan comes to mind. Meanwhile, Kristoff and Sven continue their travels into the mountain, though the latter suddenly takes a stand and pressures Kristoff to return. Kristoff's insecurities drive him to refuse when suddenly, a giant snowstorm (created by a frightened Elsa) begins to form around the castle.
Kristoff and Sven presume Anna to be in danger and rush off together to rescue her. From the castle, Olaf is able to witness the duo making their return and informs Anna. Kristoff's heroism and devotion lead Anna to believe that the former is the one she must kiss, as it is he who truly loves and cares for her. With this new spark of motivation, Anna and Olaf exit the castle and enter the fjords, just as Kristoff and Sven enter the icy platform, themselves; due to the blinding whiteout of a storm, however, each party has a difficult time reaching the other. Before long, the storm becomes deadly, creating cracks in the icy surface, revealing freezing waters. Sven tries to narrowly escape the hazards, but falls into the water and apparently drowns. Kristoff looks on in horror, but Sven survived and makes his way to safety before urging Kristoff to push forward.