"The game takes place in a fictional urban city called Metro City. As the story unfolds, the city's local superhero named Vortex is outnumbered by gangs and killed. With the city's protector murdered, the crime grows rapidly. Soon enough the city's crime lords start fighting over control of the city, until a villain named Sutekh takes control, combining all the gangs into one. With the city completely overrun by Sutekh and the other crime lords (Rat King, Goliath, Lord Muck, and Ninja Mistress), it is soon devoured in crime. A vigilante named Mark Gray (alias Nightshade) decides to step up and take the law into his own hands, vowing to rid Metro City of crime."
Source: Wikipedia, "Nightshade (NES)," available under the CC-BY-SA License.