Monster Party

The player takes the role of Mark, who wields his bat to attack and to deflect projectiles. Rebounding projectiles back at bosses is often necessary to defeat them as Mark has limited attack options. 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. The effect of Bert's beams on enemies is stronger than Mark's bat attack, and gradually improves as the player completes levels.

Monster Party features eight levels, each of which has a unique password that allows the player to access a particular level without starting the entire game over. The basic objective in each level is to beat particular bosses (and a certain number of them) to earn a key and move on to the next level. 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. Every door, whether it leads to nothing or a level boss, gives the player a question mark ('?') item, which can either provide the player with health, points, or the transformation into Bert.

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". Being the first person he encounters, Bert tries to enlist Mark as his aid in battle. Mark is reluctant to help, but Bert explains that anyone will do, and that Mark's baseball bat will be as good a weapon as any. Bert quickly whisks him away, and on the way to Dark World magically fuses himself to Mark so that they are one being (with Mark able to transform into Bert for a limited time).

The bizarre premise and plot introduction sets up a game that is filled with variations on many traditional horror characters and themes (sometimes with ironic twists), as well as relatively novel characters all its own. Some traditional horror bosses the game features include a mummy, giant spiders, zombies, Medusa, a Dragon, and The Grim Reaper. The game also features several enemies drawn from Japanese folklore and Japanese urban legends, such as Banchō Sarayashiki's well and Jinmenken (human faced dogs). 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.

Monster Party is infamous in the retro gaming community for prototype images released in Japan before the game's release in the US. The images reveal a different original concept for the game that was much darker and more graphically complex than the version eventually released.

Originally, the title screen was to be covered in blood, and the enemies designed to look much more horrific than in the final game. The first level's setting, presently an abstract black-and-pink pattern, was originally an elaborately drawn mountain range at sunset. Other changes revealed by these images is a change to the first boss, a large plant, which was shown to have a microphone and speaker in the original, a possible reference to Little Shop of Horrors.