Virizion, Terrakion, and Cobalion belong to a group of Pokémon called the Swords of Justice, which travel the world to protect people and Pokémon in need of help. Keldeo is in training to become a member of the Swords of Justice. Its final challenge before it can become part of the group is to battle Kyurem, a powerful Dragon-type Pokémon with the ability to wield the power of Reshiram and Zekrom which lives in an abandoned mine. However, the Swords of Justice do not believe that Keldeo is ready.
Keldeo sneaks out to the mine to battle Kyurem, lying to it that it is a Sword of Justice. Kyurem breaks Keldeo’s horn, causing Keldeo to become nervous and afraid. The Swords of Justice arrive at the mine to try and stop the fight. Angered by this, Kyurem freezes the Swords of Justice in ice. Keldeo runs away in panic. Kyurem chases after Keldeo, announcing that their battle is not over.
The Pokémon trainers Ash Ketchum, Iris, and Cilan find an injured Keldeo on top of their train headed for Roshan City. Kyurem arrives with its minions, several Cryogonal, still wanting to continue the fight. However, Kyurem withdraws when the train enters a tunnel. Keldeo is taken to Roshan City’s Pokémon Center to be healed. Outside, Ash, Iris, and Cilan agree to help Keldeo and free the Swords of Justice together. Iris commandeers a zeppelin from the city’s museum while Cilan uses an abandoned mine cart powered by his Stunfisk to bait Kyurem and the Cryogonal into following them while Ash and Keldeo sneak away to save the Swords of Justice on foot.
Kyurem returns to the mine to continue the battle. Ash heads to the mine with his Pikachu, Pignite, and Boldore to free the Swords of Justice. Infuriated that they are trespassing its home, Kyurem tries to attack them, but Keldeo rescues them, gaining the courage it needs to fight. Kyurem reveals that it already knew that Keldeo had lied about being a Sword of Jusťice but still accepted its challenge and intends to finish the battle. Keldeo transforms into its Resolute Form. The battle causes the mine to start collapsing, breaking the ice the Swords of Justice were imprisoned in. Kyurem encases Keldeo in ice. Keldeo breaks out of the ice after learning the move Secret Sword, in which a glowing sword emerges from its horn. Kyurem and Keldeo fire blasts at each other but Kyurem’s blast accidentally gets sent towards Ash and his friends. Keldeo slices the blast with Secret Sword, saving its friends but injuring it. Keldeo yields to Kyurem, and the battle ends.
The Swords of Justice praise Keldeo for finally learning the true power of the sword, which officially makes it the fourth member of the team. The group retreats from the mine as it collapses. Kyurem freezes the falling mine to prevent any more damage from being caused.