The player assumes the role of Darrian, an ace starpilot called into action by a mayday signal. A Federation captain is under attack by the alien Xenites, and Darrian is the only other fighter in the sector. It's up to him to clear a path through the Xenite armada singlehandedly.
The player craft has two modes, a flying plane and a ground tank. It can transform between them at almost any time during play (except when fighting a boss, as well as certain levels where a single mode is forced). The mode chosen depends on the nature of the threat the player faces. Unlike most Vertical Scrolling Shooters, the player can move in any direction instead of only straight up.
This game provides examples of:
- All There in the Manual: The game's story was only revealed in the instruction book.
- Attack Its Weak Point: Your task in Boss Battles.
- Cheat Code: On the Spectrum, pausing and holding down the keys TINY results in the message "Greetings from the mad axeman. You now have an invincible ship."
- Collision Damage: Hitting enemies will drain your shields. Hitting a boss is fatal.
- Creator Cameo: Captain Xod is Bitmap Brother Eric Matthews (digitally altered to give himself a cooler haircut).
- Mini-Boss: There's a "Sentinel" halfway through each level, and a larger one at the end.
- Nintendo Hard: Make no mistake, this game is hard to beat.
- Power-Up: Destroyed enemies sometimes drop these.
- Shout-Out: The heroic admiral who foiled an extraterrestrial spider invasion is named Ziggy S.
- Space Cadet Academy: The Panterran Ecole d'Espace (abbreviated "P.E.d'E." to thwart Fun with Acronyms).
- Suicide Attack: Some of the Xenite ships exclusively employ these.
- Temporary Invincibility: The "A" (Armour) power-up makes you invincible for 15 seconds.
- Working Title: Kelly X, after Bitmap Brother Steve Kelly. The title was changed when an incomplete Atari ST version leaked from Mastertronic (the publisher's parent company).
- A year later, for reasons unknown, Mastertronic used the title Kelly X for an unrelated ST/Amiga space shooter by a different developer. A disclaimer on the box alluded to the Brothers' home base:All characters and locations featured in this production are entirely fictitious and do not refer to anyone living, dead or working in Wapping.
- A year later, for reasons unknown, Mastertronic used the title Kelly X for an unrelated ST/Amiga space shooter by a different developer. A disclaimer on the box alluded to the Brothers' home base: