"The Secret of Cybertron" was a proposed script by writer Flint Dille and creative director Jay Bacal, hastily produced after they read Ron Friedman's original script for The Transformers: The Movie and deemed it "incoherent." Although hugely proud and excited about the script they had created, Dille and Bacal were disappointed to discover that executive producers Joe Bacal and Tom Griffin did not share their enthusiasm, and "The Secret of Cybertron" was promptly "filed away."
Details on the story of "The Secret of Cybertron" remain minimal, as the script is totally lost to the mists of time - not even Dille owns a copy. It is believed that at least some aspects of the story made it into the completed movie, although Dille describes the finished feature as "such a Frankenstein of different drafts and ideas and people" that he cannot remember which elements, if any, originated in his and Bacal's script. What is definitely known, though, is that "The Secret of Cybertron" created the idea of the Quintessons' as the creators of the Transformers, which Dille later revived for "Five Faces of Darkness".
It's also been said that the plot involved Optimus Prime sacrificing himself by merging with Cybertron and the planet transforming into a giant robot to fight Unicron, but the documents that made those claims originally are likewise lost to the universe, and no existing interviews with Dille have restated them, so whether or not they're true is not really known.