In Elsa's ice castle, Anna arrives to bring Elsa back to Arendelle. When Anna refuses to leave Elsa, who wishes to be alone, Elsa creates Marshmallow to force Anna, Kristoff, and Olaf out of the castle. Anna, however, throws a snowball at Marshmallow, provoking the snow beast who immediately grow sharp icicles all over his body and roars loudly. He then pursues the trio until they are cornered at a cliff. Marshmallow attempts to pull Kristoff and Anna back up from their escape rope. He tells them to not come back as Anna cuts the rope. This causes the two to fall, leaving Marshmallow to return to Elsa.
When Elsa's castle comes under attack by Arendelle's soldiers, Marshmallow helps protect it by smashing away the soldiers. The Duke's Henchmen shot arrows at Marshmallow, causing him to grow more icicles all over body once more, growing sharper claws and fangs. However, one of the men, Hans, slices Marshmallow's left leg off, causing him to become unbalanced. While Hans continues to the castle, Marshmallow tries to grab him, only to lose balance and plummet into the chasm surrounding the castle.
After the credits, it is revealed that Marshmallow had survived, and he's seen limping back into the palace where all he finds is Elsa's crown. The monster happily crowns himself ruler of the palace and restores his wounded leg to perfect health, happily continuing his life in the North Mountains.