[3] The story centers on a child named Mark (originally known as Hiroshi (ひろし) in the unreleased Japanese prototype) who, on his way home from a baseball game, is approached by a winged, griffin-like alien named Bert (originally known as Value (バリュー) in the unreleased Japanese prototype) who seeks assistance in ridding "evil monsters" from his realm, "Dark World".

Some traditional horror bosses the game features include a mummy, giant spiders, zombies, Medusa, a Dragon, and The Grim Reaper.

Enemies unique to the game include a large, talking caterpillar; walking pants; a giant cat that hurls kittens as projectiles; and a bouncing piece of fried shrimp.

Believing his adventures in the Dark World were a dream, he prepares to leave the house to school, only to find Bert standing before the front door with Mark's bat.

Enemies drop hearts, which replenish life, as well as pills which (for a small period of time) turn Mark into Bert, who can fly and shoot beams at a limited distance.

Bosses are accessed by doorways found in the main part of the level, which is side-scrolling, but many of these doors lead to empty rooms, adding challenge.

[6] The horror content that was permitted to remain in the game—notably large amounts of blood and the use of the word "hell"—have puzzled gamers, as Nintendo was notorious for enforcing censorship policies in the late 1980s and early 90s, when the game was released.

[1][6] In the latter 2000s, following the release of prototype images, gamers realized that an invisible platform inside the man eating plant's chamber was in fact the "deleted" speaker system, and that programmers had simply covered the sprite in black pixels rather than outright remove it.