"Donkey Kong Junior (ドンキーコングJR., Donkī Kongu Junia) is a 1982 arcade-style platform video game by Nintendo. It first appeared in arcades, and was later released for a variety of platforms, most notably the Nintendo Entertainment System. Over the course of the 1980s, it was also released for various console systems, with the form of the title abbreviated as Donkey Kong Jr. in most versions. Its eponymous star, Donkey Kong Junior, is trying to rescue his father Donkey Kong, who has been imprisoned. Donkey Kong's cage is guarded by Mario, in his only appearance as an antagonist in a video game. This game is the sequel to the video game Donkey Kong, which featured Mario as the protagonist and Junior's father as the antagonist."
Source: Wikipedia, "Donkey_Kong_Jr._(arcade_game)," available under the CC-BY-SA License.