A movie 12 calendar has revealed some important plot information concerning Arceus: Toward a Conquered Time and Space. It appears that Ash and crew will time-travel to the past (probably with the help of Dialga), which explains why Damos and Sheena were shown together in the trailers. Click the calendar image below for a larger picture. Thanks go to Bangiras for translating the synopsis.
Oddly, Heatran was originally shown in the poster featured on the calendar but has now been replaced with Arceus. Perhaps Heatran was just a stand-in for the original poster until Arceus would be revealed.
There is a legend in the beautiful town of Michiina… Long ago, Arceus, the Pokemon said to have created everything, created the “Life Jewel” by drawing on the powers of Water, Grass, Ground, Lightning, and Dragon from the 16 wells of life in its own body, and loaned it to a human. Because of that power, the ruined Michiina became fertile. However, the human broke his promise to return the “Life Jewel” to Arceus and started to attack it. Still harboring a terrible anger, Arceus went into a deep sleep. “I’ll pass down judgment on humanity when next I wake.”
At long last, the time has come for Arceus to awaken again. The protectors of Michiina’s temple, Kevin and Sheena, along with Ash’s group, journey across time and space to ancient Michiina in order to calm Arceus’ rage. What is the truth behind the legends told in Michiina? Who is this Pokemon Arceus, who’s said to have created everything? What is the point of the battle between the Pokemon known as gods? All will be made clear now, in the first ever Pokemon movie time slip adventure!