The plot in this mod involves Keen rescuing his pet Yorp from the planet Yorphius II. Unfortunately the story text included with the mod is very short and does not explain several details.
This mod, despite being before the time of many utilities and patches, has a stamp of great quality and is one of the most well-respected mods. A multi-page thread on Keen: Modding started in May 2006 was active for multiple months, discussing the level design of the mod. The levels are typically concise, well-planned scenarios and puzzles. KeenRush's style changed somewhat between this mod and Episode Smile. It is one of the small handful of mods to have a boss battle at the end.
Many have wondered the strange sounds they have heard when they have been playing the mod. The mod requires the original SOUNDS.CK1 file from the original Keen 1, and many have not noticed that (even if it is pointed out in the readme), and that has created slight confusion. The mod just does not have modified (nor corrupted) sound data. It is the absence of the SOUNDS.CK1 file what causes the game engine to read random garbage from memory and try to play it throught the PC Speaker. It is not known whether or not this can eventually crash the game, or the whole system, if running from native DOS.
In summary, the mod was designed to be installed by overwriting the contents of a folder containing a copy of the original Keen 1 files.
The original version of Yorphius II contained a modified executable, and is thus illegal. When the game was first made, CK1PATCH had just been released. Many people did not understand how to work the program at the time, and as a result the creator directly modified the executable, something that could be considered illegal by the original game authors/publishers. This is not considered much of a crime, as Keen 1 is a shareware game and free to distribute and obtain legally.
Some time later, Stealthy71088 and Levellass worked together to identify all the modifications made to the executable; they then used CK1PATCH to patch an unmodified copy of KEEN1.EXE and released a legal copy of the mod. Links to the legal copy are included below.
Vorticon Guard replacement
Tank Robot replacement
Spoiler Warning! This shows the whole levels.