Reshiram was commanded by the Hero of Truth one thousand years prior to the events of the films. At the end of their war with the Hero of Ideals and his Zekrom, it returned to the form of a Dragon Stone and was sealed away in the caves inside the Sword of the Vale. During the actual events of the films, Reshiram helps Zekrom save the world from the corrupted Dragon Force by funneling it into the Sword of the Vale. It also aids by trying to destroy the Protecting Pillars and then by helping guide the Sword of the Vale back down to the ground.
Black: Victini and Reshiram
Sometime during the three years he spent traveling the world prior to the events of the film, Damon happened to overhear Reshiram calling to him while he observed the ruined Kingdom of the Vale alongside Mannes. Together, the two of them went down into the caves inside the Sword of the Vale, where they became separated. At the bottom of the cave Damon discovered the Dragon Stone containing Reshiram and reawakened it. With Reshiram's help, Damon traveled to the villages of the People of the Vale and managed to convince them to support him. Its first appearance in the film comes when it helps Damon stop a giant tornado in the desert along with a herd of stampeding Bouffalant. It later appears in a flashback as Damon tells the story of his travels. When he raises the Sword of the Vale and comes to be opposed by Ash, Cilan, Iris, Carlita and Juanita, Damon summons Reshiram to aid him. It battles Juanita's Golurk, which Reshiram easily defeats. The second time it fights Golurk, it is interrupted by Zekrom, which Ash has discovered and allied with. The two dragons battle after Zekrom drops Ash at the castle, ending in Reshiram's defeat. Zekrom and Reshiram then realize that Damon's actions have unleashed a corrupted Dragon Force that threatens to destroy the world, and both dragons blows away the clouds to reveal this fact to Damon.
White: Victini and Zekrom
Reshiram appears in the story of the People of the Vale, but does not appear itself until Ash discovers its Dragon Stone inside the Sword of the Vale. It questions Ash about what truth he is pursuing, and when he answers that he wants to save Victini, Reshiram is satisfied in Ash's truths and appears before him as an ally. It bursts out of the cave and rescues Juanita's Golurk by stopping Damon's Zekrom's attack.