This is a Flash version of the classic 1983 Nintendo multi-screen "Game & Watch" handheld LCD game (Model No. JR-55). You (playing as Donkey Kong Junior) must collect 4 keys to free Donkey Kong from Mario's chains.
The game will start as soon as it has finished loading. Use all 4 Cursor keys (Arrow keys) to move Donkey Kong Jr., and the Space Bar to jump. Click on the game to activate the control keys. Whenever the game ends, press the F5 key to re-start it.