In my opinion, is THE greatest, next generation Pokemon movie. Enjoy:
Actual Movie
Ash arrives at Cameron Palace where a festival is being held to celebrate Sir Aaron, an ancient hero who averted a war that was about to begin outside of the Tree of Beginning. At the festival, Ash competes in a tournament at Cameron Palace and wins, to become the "Aura Guardian" for that year. As part of the celebration, Ash is granted Aaron's staff, which contains his Pok?on companion, Lucario, whom Aaron had sealed before stopping the war. However, Pikachu is taken away by Mew when an adventurer named Kidd attempts to put a tracker on the legendary Pok?on using her Weavile. Naturally Ash, with the help of Lucario, follows Mew to the Tree of Beginning in order to rescue Pikachu. There is but one problem. After being sealed in the staff, Lucario has completely lost his trust for humans. Then Ash earns Lucario's trust and they enter the Tree of Beginning. They are attacked by Regirock, Regice, and Registeel, the tree's guardians who recognize them as a threat. They enter the tree and are attacked by the tree's defense system, white blood cell type mechanisms, because of Kidd. The white blood cells are able to transform into fossil Pok?on like Aerodactyl and then absorb the threat. When Ash and the gang are absorbed into the tree, Mew saves them by reasoning with the tree's defense mechanism, but becomes incredibly weak in the process. In order to save Mew and the tree, Lucario and Ash combine their Aura to reverse the self-destruction of the tree. Lucario pushes Ash away towards the completion of the process so that Ash won't end up forfeiting his soul to the realm of Aura. Afterwards, a "time flower" shows a memory of Aaron sacrificing himself to stop a war. Right before Aaron died (in the memory), he said how Lucario was his closest friend and would miss him, and then he died, it is made clear that the reason Aaron sealed Lucario was to ensure that Lucario didn't die with him. Sadly, it means that Lucario dies and moves on with Aaron in the next life, his last words are "You were my best friend too". The dramatic ending concludes the story well as all characters move on after learning important lessons of trust, sacrifice, and love.
Ending
Even though (according to the movie) Lucario sacrificed himself in order to save the Tree Of Beginning, whether he truly died is unknown, since at the end of the credits, he and Sir Aaron appear together eating a chocolate bar. However, this could be explained that they are sitting enjoying the afterlife, or that they didn't truly die. I believe that this was a flashback.