Ash and Goh travel to the Sinnoh region to investigate after learning that people are unable to sleep due to nightmares. The two of them suspect that a Darkrai is causing the nightmares. Meanwhile, Chloe travels to Sinnoh by herself and eventually meets Ash's former travelling companion, Dawn. As Ash and Goh try to search for Darkrai, they realize that Cresselia is able to prevent nightmares caused by Darkrai. In a forest, Chloe and Dawn find an injured Cresselia. (Source: Wikipedia)
