[3] Once entered, the only way to exit a level is to reach the end, though unlike in the first trilogy of episodes, Commander Keen in Invasion of the Vorticons, the player can save their game at any point.

In the first episode, having defeated the Grand Intellect in Invasion of the Vorticons and saved the Earth, eight-year-old child Billy Blaze is building a faster-than-light communications radio.

Donning his helmet as Commander Keen, he takes off in his spaceship—first stunning his parents with his neural stunner gun as they call him in to dinner—for the planet of Gnosticus IV, home of the Oracle and the Gnosticenes that tend it, to find out who the Shikadi are and how they plan to destroy the galaxy.

In October–December 1990, a team of employees from programming studio Softdisk, calling themselves Ideas from the Deep, developed the three-part video game Commander Keen in Invasion of the Vorticons.

Keen's red sneakers and Green Bay Packers football helmet were items Hall wore as a child, dead enemies left behind corpses due to his belief that child players should be taught that death had permanent consequences, and enemies were based loosely on his reading of Sigmund Freud's psychological theories, such as that of the id.

[7] The team reprised their roles for both Keen Dreams and Goodbye, Galaxy, with the addition of programmer Jason Blochowiak for Goodbye, Galaxy, and changed the game engine and design for the next Keen games: an increase in graphical quality, a pseudo-3D view rather than a side-on view, ramps rather than solely flat surfaces, support for sound cards, and changes to the design based on player feedback.

[5][1] The level maps were designed using a custom-made program called Tile Editor (TEd), which was first created for Dangerous Dave and was used for the entire Keen series as well as several other games.

[9] For Vorticons, Carmack had created adaptive tile refresh to produce a scrolling effect on computers not powerful enough to redraw the entire screen when the player moved.

By August they had completed a beta version of episode four, "Secret of the Oracle", and Romero sent it off to a fan he had met from Canada, Mark Rein, who had offered to play-test the game.

Within a few weeks of being hired, Rein made a deal to get id into the commercial market: to take the sixth episode and make it a stand-alone game, published as a retail title through FormGen instead of part of a shareware trilogy.

Id signed the deal, but Scott Miller of Apogee was dismayed; he felt that not having a full trilogy for the shareware game would hurt sales.

There, they worked on Goodbye, Galaxy, their remaining Softdisk games, and the now standalone Commander Keen in Aliens Ate My Babysitter between August and December.

[13] Despite being listed numerically as the sixth episode, because Aliens Ate My Babysitter had a different publisher and schedule it was developed after "Secret of the Oracle" but before "The Armageddon Machine".

Scott Miller of Apogee blamed the falling sales on the lack of a third episode, which he felt undercut the shareware model of the game.

Acknowledging its debt to Super Mario Bros., he called it, especially Goodbye, Galaxy, "one of the best games of its type" and praised that it was not "mindlessly hard", instead requiring some thought to play through, and especially the humor in the graphics and gameplay.

[2] Id Software did not produce any more games in the Commander Keen series after Goodbye Galaxy besides the co-developed Aliens Ate My Babysitter.

[1] Another trilogy of episodes, titled The Universe Is Toast, was planned for December 1992, but was cancelled after the success of id's Wolfenstein 3D and development focus on Doom.

When GT Interactive wanted to publish Doom II, it proved itself to id by quickly selling 30,000 copies of Goodbye Galaxy.