Andrew Rannells - James
Lauren Kling - Jessie
Darren Dunstan - Giovanni
Kathleen Roche - Meowth
Dominic Nolfi - Ash
Heidi Michelle Weyhmueller - Misty
Dennis Kenney - Brock
Team Rocket, of course, steals the show. They have three songs, "It Will All Be Mine", and "The Best At Being the Worst", and the classic "Double Trouble". All very nicely done songs, yet "It Will All Be Mine" sung by Giovanni is my absolute favorite.
The basic story is that Giovanni has created a mechanical Mewtwo, inspired by the real Mewtwo. He wants to rule the world with it, like all good villains. Ash gets involved, there's battling and Ash wins. All filled with songs. There's a little side story involving Ash's mother's past relationship with Giovanni and Misty's feelings for Ash but besides that, it pretty much runs as a regular Pokemon episode.
Now, just a couple pictures. Courtesy of the official site. ^_^
From 'The Best At Being The Worst'
From 'Double Trouble'
And some from who knows where.