Directly from steam:
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Prologue features roughly 1/3rd of the main story (individual girl's routes not taken into account).
It serves as an introductory, standalone, experience before the final release of the game (that won't be free and won't be finished for a while).
I've been working on Monster Girl Island for 3+ years, it's been a journey, but after much learning and many re-doings I finally think the game is polished enough for the public. What you see here is what you'll get in the full release, for the most part. So please look forward to Monster Girl Island if you enjoy this first part!
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The full release will be called "Monster Girl Island" and has not yet been released.