I haven't seen it but this is from wikipedia:
Plot
Nishi is a young loser who wishes to be a comic book artist. He has always had a crush on his childhood friend Myon, an enormously-endowed young girl. They lose contact spanning a period of time up until Myon is to be married. When they meet each other again and they go to her parent's restaurant, to see Myon's father and sister and talk about old times, a problem arises when two yakuza enter the restaurant asking for money that Myon's father owes them.
Nishi gets killed in a shameful way, the yakuza goon winds up shoving his gun up Nishi's rear end and pulling the trigger, and goes to heaven, where he argues with God. He then manages to escape, go back in time, rescue Myon and her sister and run away from the yakuza stealing their car. After a high speed chase and a consequent jump off a bridge their car gets swallowed by a whale. During the symbolic and metaphorical time inside the cetacea (there is a reference to Carlo Lorenzini's Pinocchio), they experience personal development and growth and have to face their problems and insecurities.