The Snow Queen

Ellie and her brother Tom listen to their grandmother reading them a story about the evil Snow Queen. When their younger sister Polly asks Tom if the Queen is coming to their house, Tom says that she only exists in the story. However, the Snow Queen really does live in an icy palace in the North Pole with her three troll servants: Eric, Baggy, and Wardrobe. Her plan is to set up her huge magic mirror on a mountain to reflect the sunlight away so the entire world will become her kingdom. But the mirror falls down the mountain and lands on the trolls' flying machine where it shatters into pieces. Two of the pieces hit Tom in the eye and the heart that he falls into an evil curse.
The Snow Queen sends her bats to retrieve the pieces. As they cannot take the two that are inside Tom, the Queen goes out to kidnap him. Ellie and Tom connect their sleds to a bigger sled that is revealed to be driven by the Snow Queen. She takes Tom to her palace and cuts Ellie off of her sled, causing her to fall onto a talking bird named Peeps. Ellie goes out to save Tom from her and Peeps reluctantly decides to go with her. In a snowy forest, they find a house belonging to an old woman, who appears nice, but is actually an evil witch but traps them to use Ellie's heart for her elixir of life so she can be eternally young. Peeps tricks the witch's cat, Cuddles, into chasing after him and knocking over the elixir of life, and uses the confusion to unlocks Ellie's cell. Ellie and Peeps escape and trap the witch in the basement.
They then meet two humanoid birds named Les and Ivy, who, from Ellie's description of Tom, tell her that Tom is going to marry the princess, so Ellie becomes a member of the staff to serve the princess her food. However, she soon discovers that the prince is not Tom. Meanwhile, Tom is rebuilding the Snow Queen's mirror, as he is good at puzzles. The trolls try to warn him that the Queen is going to kill him to get the last two pieces, but the Queen convinces him otherwise and kisses him, putting him into a hypnotic state while his veins are full of ice, and will cause his death when it reaches his heart.
The princess and the prince give Ellie and Peeps a royal vehicle to ride to the Snow Queen's dominion, however they run into a robber gang of humanoid rats. The Robber King promises his daughter, Angorra, that Ellie can become her slave, but later changes his mind. Ellie is locked in a room with a flying reindeer Dimly who was captured by the robbers. Peeps enters the room and unties Ellie's hands, and she unties Dimly. Angorra enters, but they trap her with a barrel. Dimly flies them away, but the King grabs onto the rope that is still wrapped around Dimly, resulting in the King slamming into a building and falling over the edge on top of Angorra.
Dimly does not know where the Snow Queen is, so he goes to his flying reindeer school and asks Freda, a Lapland woman who runs the school. Freda has Dimly fly them over to the Queen's castle. There, they meet the three trolls, who ultimately decide to help them. Tom does not have much time left, and has finished putting the mirror together except for the two pieces that are inside him. Freda reveals that the two pieces inside him will kill him, then makes a potion that will dissolve the mirror. Ellie tells Tom to drink it, but just as he is about to, the Queen blasts the vial away with her magic staff. They fight the Queen but Eric and Freda get frozen, and Baggy and Wardrobe grab her staff just as they are frozen as well. The battle eventually causes the vial to fall on top of the mirror and shatter, dissolving the mirror and forming an icy cyclone that chases after the Queen's flying carriage and freezes her solid as she attempts to escape. The mirror pieces inside Tom dissolve and the effects of the Queen's kiss go away, freeing him. Freda and the trolls are unfrozen.
Freda warns the Queen is not dead and might return in the future. She has Dimly take Ellie, Tom, and Peeps back to the village, and then come back for her and the trolls. Dimly crashlands in the village and Ellie, Tom, and Peeps go to listen to the rest of the story as Dimly heads back to the Snow Queen's palace. The film ends with a close-up shot of the frozen Queen's eyes lighting up.
Musical numbers: Welcome To My World; Recommended By Me; Open Road; Down And Dirty; Flying School

Question: What is tom rebuilding ?
Answer: Tom is rebuilding the snow queen's mirror.