Like most such games the story is basically an Excuse Plot: in the wake of World War III, Earth is invaded by aliens called the Kretons. The initial Earth counteroffensive is a disaster, and the majority of Earth's soldiers are capture. Earth's last hope is the hero of WWIII, Major Stryker, who is sent through the wormhole the Kretons came through to strike back.
Each planet consists of four zones, each consisting of two levels with a theme plus a boss. All the bosses in the game have different designs and the three end-of-episode bosses are all multi-stage.
The game features PC Speaker and Sound Blaster sounds, and Ad Lib music similar in style to Bobby Prince's other songs; it was the first game published by Apogee to include the famous Apogee Fanfare. The game uses EGA graphics and draws three backgrounds ("triple-parallax scrolling").
This game contains examples of:
- Alien Invasion: The Kreton timed their invasion just right as the Earth just got off WWIII.
- Aliens Are Bastards: The Kretons seem to be a collection of different aliens species bent on conquering the Earth.
- Asteroids Monster: Each episode has a boss that splits into two smaller versions of itself when you've damaged it enough.
- Boss-Only Level: All the bosses get their own level.
- Excuse Plot: It's covered in about four screens of text and pictures, and considering the resolution of 320*200, you're not going to fit much text on one screen.
- Gatling Good: Gatling cannons are usually found in pairs, most of the time on Bosses' ships.
- Hypocrite: One Boss alien angrily calls you out for destroying his city. Really, who attacked the Earth in the first place?
- Insectoid Aliens: One alien Boss look like an insect with three eyes while another has two.
- Invincibility Power-Up: The bubble, which makes you invincible and able to defeat enemies by ramming them.
- Lizard Folk: One alien Boss look like a lizardman.
- Mission Control: Fleet Admiral Yoshira briefs you before entering each zone. It's mostly just for flavor, as your goal is always the same: get through two stages and then defeat the boss.
- One-Hit Point Wonder: With no weapon upgrades, one shot and you're toast. With them, you can take one hit without dying, at the cost of the weapon upgrades.
- Recurring Boss Template: There are three such templates, each used once in each episode, though with differences. For instance, each episode has a boss which splits into two smaller copies of itself for its second phase.
- Shout-Out: One Boss's face looks like a Stormtrooper from a galaxy far, far away.
- Single-Biome Planet: Strangely subverted. The three planets are named "Lava Planet", "Arctic Planet", and "Desert Planet", but the stages reveal each planet has a mix of biomes. For instance, the lava planet has oceans and temperate grasslands with cities.
- Smart Bomb: Called "Zap Bombs", they deal 2 Hit Points of damage to all enemies on screen, and eliminate all projectiles.
- Your Princess Is in Another Castle!: At the end of the first two episodes, Yoshira tells you that the planet you just defeated isn't the Kretons' "stronghold", so you should head to another planet.