First, you start off as Dr. Jekyll, walking and walking (and walking) though the town, bridges, and forest to reach your destination. (I haven't read the book, so I cannot give the similarities from the book to the game.) You have to watch your meter as well as your life bar, but mainly your "rage" meter. What's very strange is whom you should avoid to keep Dr. Jekyll sane. You would have to avoid number of people as they may attack or plant bombs that seems impossible to avoid. You also have to avoid cats, the god-awful fat lady singer, to the bird constantly, err... dropping. The only way you could avoid getting mulled, you can only do four things, jump, poke with your cane, enter doors, or run like hell.
If the meter reaches down to "H," you will transform into the hideous Mr. Hyde, a.k.a, bad guy.
Now onto the Mr. Hyde part of the story. You play the exact same level design as before, but in reverse, in the night, and with monsters. On a somewhat plus side, you can shoot fireballs at the monsters. You have to defeat them in order to collect coins and increase the meter back to J to turn back into "Dr. Jekyll" again. If you don't and happen to stumble to the same position as you left off when you turned into Mr. Hyde, a deadly bolt of lightning will strike you and you will die, game over. Sounds fun, right?