The movie starts off with a silver haired boy lying deep beneath the iced-over surface of a pond. Suddenly, the surface cracks and the boy emerges from the water. At the same time, there is a voice-over saying that he does not know how he got there, but the moon was responsible for his reawakening. This boy is known as Jack Frost (Chris Pine). Whatever he touches turns to ice and frost, and he can sort of "fly" by manipulating the windy drafts to carry him about. Jack also finds a wooden staff which he is seen carrying around the rest of the movie.
Using his ability to fly about, he flies to the nearest village, making a big disturbance as he lands. However, despite his enthusiasm to meet the villagers, no one sees him, and they walk through him, as though he is a ghost. He is disappointed and flies away.
Cut to 300 years later, a Guardian (Actually Santa Claus) named North (Alec Baldwin) is introduced and seen carving an ice train to play. (Throughout the movie, Santa has this weird foreign accent) Suddenly, one of his bear helpers interrupt to tell him that there is a problem with the "Globe." The Globe is a model of the entire world and shows little yellow twinkles which represent the children of the world. When a child stops believing in these legendary creatures (Santa Claus, Easter Bunny, Tooth Fairy) the light goes off. However, as represented by a swirl of black sand around the Globe, a threatening force known as "Pitch" aka the Boogeyman has the intention of quelling all of children's dreams and joy found through these fantasies and instead choosing to propagate evil and fear throughout. North thinks this to be a serious problem and calls an emergency meeting of the other Guardians, which are then introduced. They are the Easter Bunny (Hugh Jackman) (who has the ability to create portals through the ground, also responsible for producing Easter eggs), Tooth Fairy (Isla Fisher) (who, together with her other helpers, collect baby teeth which contain childhood memories) and Sandman (called Sandy, uses his magic golden sand to create happy dreams for children). A more troubling matter for the team is that Pitch (Jude Law) has managed to use his evil powers to transform Sandman's magic golden sand into a black sand which Pitch can manipulate to cause nightmares in children.
They gather around and decide how to fight Pitch. However, the Moon (shown to be some sort of powerful force in the movie--an allegory to God somehow) shows them that they need a new Guardian to assist them, revealed to be Jack Frost. This is met with some unpopular reactions, as Jack is thought to be unreliable. Worse still, Bunny has some sort of personal vendetta against him, as one Easter in '68, Jack had frozen the burrows with which Bunny used to transport his Easter eggs. However, they decide it needs to be done and thus go to pursue him.
Cut to a scene with Jack causing mischief in the house of a little human boy, Jamie (Dakota Goyo), by using his frosty powers to make Jamie's sled travel at high speed around the town, and causing snowball fights. The point is, Jack, having been alone and idle for years, is misusing his powers for fun. He lacks purpose in life and constantly questions why he lives.
Suddenly, he is captured by two of North's bear helpers and brought to the headquarters where he is initially reluctant to be a Guardian. However, he decides to join them temporarily when baited with the prospect of having the chance to ride on Santa's sleigh.
They travel to Tooth Palace (lair of the Tooth Fairy) where they discover that children's teeth, which had been carefully collected by fairies, are being stolen by Pitch, including the fairy helpers who are kidnapped. Tooth (the head fairy) is very upset as these teeth are valuable due to their containment of memory. She mentions that Jack has teeth collected by them, causing him to realise that perhaps, he had a human life before, but cannot remember. Thus, he aims to join the Guardians, if only to regain his past memories.
However, at the same time, Pitch is succeeding in his evil aims. The fairies that he captured are unable to perform their duties (collecting teeth and leaving rewards), and thus children round the world are losing belief in the Guardians, and thus they will grow weaker eventually. The Guardians thus embark on a mission to collect teeth and leave rewards.
At Jamie's house, however, when removing his tooth (Jamie gets a tooth knocked out in the earlier sled race), the Guardians accidentally wake Jamie up. Thus, his belief in these creatures is once again fuelled. However, his younger sister Sophie (Georgie Grieve) wanders into a portal created by a random snow globe that rolls out of North's pocket. No one notices as Sandman, who had been trying to get Jamie to sleep, accidentally dusts his magic golden sand over everyone except Jack and they all fall asleep.
Suddenly, Pitch appears, and Jack pursues him quickly. However, Pitch has his army of black sand creatures which Sandman fights with his good golden sand. Alas, he is overpowered and is apparently killed. Jack, very angry and upset, uses his frosty magical staff to freeze all of Pitch's minions. Pitch is chased away - for now.
The Guardians are all very upset that Sandman has died. Jack in particular is especially upset as he feels extremely powerless. However, as Easter is upcoming, the Guardians proceed to the burrows of Bunny to prepare. Magical eggs there are painted (they have feet, which are able to transport themselves to the human world where they can be found during hunts). Suddenly, they discover Sophie, who had earlier been teleported here. Jack volunteers to bring her back home.
Tucking her in, they start on their journey back. However, Jack hears someone calling his name. However, it is a ploy by Pitch to lure him to the underworld. There, he is locked in a tunnel where he discovers that all the eggs meant for Easter have been crushed while travelling to the aboveground. In addition, Pitch has placed his baby teeth in the pocket of his hoodie.
Somehow, he finds himself back with the Guardians, looking very dejected as Easter has been ruined. In addition, children have begun losing their faith in the Bunny as they can't find any eggs. Hence, Bunny becomes invisible to them. Suddenly, Tooth spots the golden canister containing Jack's teeth in his hand. They all feel betrayed as they believe that Jack was in cahoots all this time with Pitch just to get his memories. They separate.
Jack angrily storms away to an icy mountain. There, he is tempted by Pitch to join him as they have something in common of being hated by people and always alone. However, Jack is not convinced by Pitch's goal of uniting to spread fear to children. Pitch is angered and throws Jack into a snow crevice, breaking his staff, as well.
Feeling hopeless, Jack opens up the canister containing his memories, showing him as a brown haired boy 300 years ago. He was still human then, bringing his sister to a nearby lake to ice skate. However, the ice is thin, and Jack desperately tries to save his sister as the ice cracks bit by bit. Disregarding his own fear, he engages his sister in an innocent game to get her off the ice, even though he is in a dangerous position. He succeeds in getting his sister off the thin ice; however, he falls in. This is how he eventually gets to be under the ice and later, becomes Jack Frost, as chosen by the Moon.
Knowing this, he then realises that indeed he is Guardian material and chosen for a reason. He heads back to the tunnel where he first encountered Pitch. Pitch has his own version of the Globe, which only shows one twinkling light, revealing that there is only one kid left who still believes in them. Jack rushes to Jamie and manages to convince him that the Easter Bunny exists by drawing on a frosty window.
Thus, Jamie is now convinced that such mystical beings exist and begins to see Jack, fulfilling his dream of being noticed.
Jack encounters the other Guardians, who believe his story. They aim to work together to defeat Pitch. Hustling together the rest of Jamie's friends and neighbours they fight against Pitch. Suddenly, Sandman, revitalised by the children's belief, comes back to life and defeats Pitch by whipping him around with golden sand ropes.
The children with their belief that Pitch has been defeated, no longer fear him and believe in him. Thus, Pitch is weakened and is eventually banished to an empty well by his minions. Evil disappears, and peace prevails.
The Guardians (Jack rejoins them) travel back to North's headquarters, knowing that hope and belief in them is restored.
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