It takes place in the event that Aron's Ending at the end of Phantasy Star III was the "true ending"
Here is a teaser:
http://www.youtube.com/watch?v=2iexHNcdJ3Y
Some more info about this in a thread over at PSO-World:
http://www.pso-world.com/forums/showthr ... 870&page=3
We should bring those film makers here...