If it's a classic with Cliff notes available, read the notes too. They'll give you sketches of the major characters and the action of each scene. I psychoanalyze the story and look for symbolic structures to find the heart of the tale. What you recall is what you should use to adapt. If you don't remember a part, maybe nobody else will miss it.
Many stories have the three-part spine underneath all the subplots and diversions.
Try to isolate it. Over the millennia, audiences have been groomed to expect stories with three-part structures and you would do well to fulfill their expectations. I'm never so bald about this, because I've internalized the common Western notions of story telling. See Aristotle's Poetics for a start of these essential elements. In the late 1980's, here in Hollywood, Joseph Campbell's 1948 classic The Hero with a Thousand Faces sparked a boom in using myths as a guide.