Dragonball Z: Secret of the Kai

First chapter is really just a trailer for it. Or Pilot in this case. When Kibito Kai shows up heavily injured claiming to have been attacked by the Elder Kai, Gohan finds himself on a journey to find out how the Elder Kai obtained this power. On this journey he will discover the answer to a question that has haunted him. Why is his Mystic state weaker than the Super Saiyan 3?