This game is quite good, but there's no need downloading if you already have Adventure Island.
See, the original arcade version of Wonder Boy was developed by a company called Escape. When it came to console conversions, Sega gained the rights to the Wonder Boy name and characters while Escape retained the rights to the game. So when Hudson Soft decided to make a NES version, they released Adventure Island, which was pretty much Wonder Boy with a different name and slightly different-looking character.
The character is called Tom-Tom and his girlfriend has been captured by a monster. So you must run and jump through forests, hills, oceans, caves, ice palaces and cloud levels in a quest to rescue her. Your stone hammer will help you destroy enemies and you can rush through the levels on a skateboard.
It's a great little platformer and if you don't have Adventure Island we do prefer this version slightly thanks to its graphical style.