Dialga first appeared in the tenth Pokémon movie, The Rise of Darkrai where it was battling against it's enemy, Palkia. During the battle, it used Roar of Time on Palkia which wounded it's left arm and caused it to flee into Alamos Town. Due to Palkia's arrival, the Pokémon who were having nightmares under the influence of Darkrai's Dark Void woke up. Simultaneously, Palkia's presence causes the dimension Dialga and Palkia were fighting in to disintergrate the normal dimension little by little. Dialga, who was furious at Palkia for its cowardice, engaged it to continue the battle which did enormous damage to the entire town. During the battle, Darkrai tried to hold them off as long as he could, but ended up getting himself killed in the process of saving the town. Before Dialga and Palkia can deliver the final blow against each other, Ash and Dawn successfully managed to magnify the volume of the song, Oración, soothing both Dialga and Palkia in mid-flight and healing Palkia's wound. Just as Dialga returned to its dimension, Ash and Dawn asked Palkia to return Alamos Town back to its original dimension. Once everything was restored, Palkia returned to its dimension.
In Giratina and the Sky Warrior, Dialga played a small role. In the beginning of the film, it appeared near a lake and gets a drink from it. But little did Dialga know, Giratina's Reverse World was infected with dark and poisonous clouds which were caused by its previous battle against Palkia from the preivous movie. The influence from the battle angered Giratina and made it believe that both Dialga and Palkia are the cause of the Reverse World's devastation. Giratina lures Dialga into the Reverse World and engages it battle. Thankfully, Shaymin's Seed Flare gave Dialga a chance to escape the Reverse World and return to its dimension.
Dialga made its last and final appearance in Arceus and the Jewel of Life. It was summoned by Sheena to rescue Pikachu and Piplup from being sucked into a deadly spatial distortion and closes it up with Roar of Time. Moments later, Giratina arrives and tries to battle Dialga. Sheena was unable to reach into Giratina's heart due to its anger. Ash successfully calmed it down by pleading it to stop after recognizing him as the same person that saved it from Zero from the previous film. After Sheena reached into Giratina's heart and told it to return to its dimension, the special distortion opens again and this time Dialga gets itself caught in the water spouts. Just as all hope seemed lost for Dialga, Palkia arrived and rescued Dialga by using Spacial Rend on the spacial distortion to close it. Sheena then connected with the hearts of both Dialga and Palkia and puts an end to their conflict with each other once and for all. After that, Dialga and Palkia returned to their respective dimensions. Sheena and Kevin explained to Ash and his friends that the reason why Dialga and Palkia clashed in Alamos Town was because both of their dimensions, which were not meant to ever intersect, intersected due to Arceus's preparation to awaken from its long, eternal slumber. As a result, both Dialga and Palkia believed that they both invaded each other's territory and the battle also affected the Reverse World which made it bring Giratina into the conflict as well. When Arceus arrived and prepared to destroy all of the humans due to Damos' betrayal, Dialga, Palkia, and Giratina arrived and protected Ash and his friends from Arceus's attacks. As Palkia and Giratina holds Arceus off, Dialga used its temporal abilities to send Sheena, Ash, Dawn, Brock, Pikachu, and Piplup back in time to give them a chance to change the past and save the future. Dialga was then badly injured by Arceus alongside Palkia and Giratina, and becomes exhausted due to time travel. Eventually, when Ash and the others had successfully changed the timeline and saved the future, Arceus healed Dialga and the others and Dialga returned to its own dimension to live peacefully once again.