Undead Line is a top-down arcade shooter with scrolling screens, like Commando and other games. The player controls a character (from a selection of three on the MSX 2 version) that shoots projectiles at enemy monsters.

The six levels available are presented in a menu and can be selected and played in any order. There is a boss battle at the end of each one.

Chests with power-ups are scattered throughout the levels. They open when shot, and in the Genesis version it is possible to cycle through the available power-ups by keeping firing at them. The power-ups include armor and speed boosts, and several types of projectiles.

In the beginning, the world was inhabited by gods and giants who fought over which race would rule. To win, the giants created four "living weapons" who were so fearsome and effective, both sides lost. The day of the monsters's creations would come to be called Ragnarök. Ages later, Count Brahzen, a leader of demons, invaded the Gitane Kingdom, who were able to defend themselves solely by tactical decisions made by their King Fahrenheit II... only to discover Count Brahzen is seeking the Spirit of Roshufa in order to revive one of the four legendary monsters. You are Leon, a fighter chosen by the King to obtain the Spirit before Count Brahzen does.