The game's story is based on a mini-anime series, with extracts from the anime being used for cutscenes between the levels. The story centers on a man named Ryo, who begins working for a company called Yaesu as a replacement for his friend Masato. During a test ride with a new machine, however, he is somehow thrown into another dimension. After being rescued there by a group of mecha pilots from vile creatures named Devastators, he learns that nefarious people are working on a connection between that and his own dimension, and sets out to stop them.
Devastator is a run & gun-style platformer with shoot-'em-up segments. The player controls a humanoid mech with a mid-range weapon, which can be described as a sickle on a chain, and one of three different long-range weapons. The goal in each level is simply to reach the end and defeat a boss. Most of levels consist of platforming, but sometimes the gameplay switches to a horizontally scrolling shoot-'em-up. Weapons can be upgraded by collecting items left by defeated enemies, similar to the Turrican games, but the protagonist may only have one long-range weapon; picking up another replaces the old one. There is only one type of standard upgrade item. To select a weapon, the player has to shoot the power-up until it shows the correct symbol.