The official Pokémon movie website has updated. Translations follow:

Story

To all Pokémon fans:
This is the story of the duo's "meeting" and "promise".
This is the creation of the 20th anniversary movie!

Pocket Monsters, or "Pokémon" for short:
Mysterious lifeforms that hide many secrets.

There are still many things we don't know about their ecology.

Even today,
Pokémon Trainers continue their adventures
with the aim of encountering Pokémon never before seen...

Satoshi, a boy living in Masara Town, greeted the morning of his 10th birthday, the day he could become qualified to become a Pokémon Trainer. Satoshi's heart was filled with a desire for Pokémon he has never seen before and a world he has never seen before, but while he was meant to be given a Pokémon that would become his partner at the Okido Laboratories, he ended up severely oversleeping, and all that was left for him was a single Pikachu, a Pokémon that did not get attached to humans.
"Do you hate me? I like you!" Even though the two were at odds with one another, their friendship deepened little by little, and as they looked up at the legendary Pokémon Houou that flew through the sky on the day they set off, Satoshi and Pikachu swore an oath as they held one of its Rainbow Feathers:
"Let's go meet it together one day!"

And thus, Pikachu and Satoshi, who set off on his journey to become the world's greatest Pokémon Master, end up meeting the Trainers Makoto and Soji on their way before getting told a legend concerning Houou.
"Those who, guided by the Rainbow Feather, meet with Houou, shall become the Heroes of the Rainbow."
Satoshi and the others end up getting guided by Marshadow, a mysterious Pokémon that had been lurking in Satoshi's shadow without anyone realizing, and make their way to Mount Tensei, where Houou lives, but a strong opponent appears before them. Will the duo manage to reach Houou?! In order for them to fulfill the promise they swore on the day the departed, the two of them now begin an adventure that will last until they become "true partners"!

Character Profiles

Characters profiles and voice actor comments
	SATOSHI (Partner Pokémon: Pikachu) A boy aiming to become the world's greatest Pokémon Master. He's somewhat of a scatterbrain, but has an honest heart. Voice: Rica Matsumoto Here it is! The 20th anniversary of the Pokémon movies!! As I have kept playing the role of Satoshi, I have kept wishing that Pokémon can become the tie that binds families together all across Japan, and this year, I believe that dream will finally come true. I am extremely grateful towards Pokémon for keeping me going for 20 years. Thank you!
	SOJI (Partner Pokémon: Lucario) A young male Trainer aiming to become a Pokémon Professor. His catchphrase is "Be advised that...". Voice: Kanata Hongo I've loved the Pokémon movies since I was a kid, so I was super happy to receive an offer to be in one of them! Not only that, but it is an honor to be in this commemorative 20th movie that not just children, but also those from my generation that are just as crazy about Pokémon as I am, will be able to enjoy. I will do my absolute utmost to make you like Soji once you're done watching the movie!
	MAKOTO (Partner Pokémon: Pochama) An energetic and masculine girl Trainer. She appears to enjoy adventures, but the truth is... Voice: Shiori Sato I remember seeing the first movie in the theater. Being able to appear as a guest voice actress in one of these Pokémon movies is like a dream for me! Not only that, but this year's movie is a tale that starts with Satoshi and Pikachu's meeting, which I saw on TV when I was little... I will do my best to play the role of Makoto, whose mental age might be even higher than my adult self.
	BONJI A researcher that has chased after Houou for a long time. The author of "My Life is Houou". Voice: Arata Furuta When I was offered the role, I wondered if I was going to play a Pokémon, but nope, it was actually the role of a human. I hope I will manage to guide Satoshi and the others to Houou while being careful to not get in the way of their adventure. I will do my best to excite both children and the adults that used to be crazy about Pokémon.
	JOY A nurse that treats Pokémon at the Pokémon Center. Voice: Shoko Nakagawa The Pokémon movies have given me lots of memories, and this is the 20th one at that, so I'm extremely happy to be in it. I want to use kindness and love to heal everyone, just like Joy, the nurse everyone admires. I will do my best to make lots of people go see this year's Pokémon movie, which is packed to the brim with both nostalgia and freshness!
	CROSS (Partner Pokémon: Gaogaen) A boy that believes strength is everything and that aims to become the ultimate Trainer.
	MARSHADOW A mythical Pokémon that appeared from Satoshi's shadow, as in response to the Rainbow Feather. Voice: Koichi Yamadera Even though I have acted in the Pokémon movies since the very first one, I approach each and every one of them with a fresh attitude. I may have moved on from "Oha Suta", which started at the same time Pokémon did, before I had been with it for 20 years, but I'm very happy to be involved with the 20th Pokémon movie. I believe this will be a memorable movie packed with all kinds of people's emotions, so I want you to have high expectations.
	HOUOU It is said that it will appear before Trainers with just hearts with its seven-colored wings shining. Species: Rainbow-colored Pokémon Height: 3.8m Weight: 199.0 kg

Teaser and trailer

Teaser 3:

Trailer 2:

Source: http://www.pokemon-movie.jp/story/, http://www.pokemon-movie.jp/chara/ and http://www.pokemon-movie.jp/trailer