Commander Keen 6 is the last of the official Commander Keens and does not exactly continue the stories of 4 and 5. Molly, Billy Blaze's babysitter, has been kidnapped by the Bloogs of Fribbulus Xax. Commander Keen must embark on a mission to save her from these nefarious and rather dim-witted creatures.
With updated graphics, gameplay, and difficulty, Commander Keen 6 makes a good claim of being the best of the series. Its rather convoluted development cycle, involving FormGen, means that it is difficult to find full legal copies, as 6 is never distributed with the other 5.