A plot description that doesn't use details from the book it is describing will, of necessity, have to be short. You can talk about the general points of exposition and what main characters are introduced at what point in the story development. You can talk about the general pattern of rising action. You can talk about complications and the climax. You can also talk about any mini climaxes along the way and about any anticlimactic points in the story. You can describe the general trend in the falling action. You can say that the resolution occurs with or without joy to the characters (in The Great Gatsby, it is without joy). You can give a general account of the Epilogue, saying whether any major character benefited from the adventure in the story. An extremely brief example of some this might be as follows:
The Great Gatsby tells the story of Nick Carroways' move to work in New York City. He encounters new experiences there and at his new home in Long Island, New York, where his neighbor is the famous and imposing Gatsby who nightly entertains as many of the rich and famous as can crowd into his palatial home. As they become acquainted, Gatsby asks Nick to do him a favor by arranging a meeting with Nick's cousin, Daisy Buchanan. This favor, which Nick succeeds in arranging, leads to joy, anger, murder, estrangement and more death. Nick, no longer naive, leaves New York for the quieter environs of his own home in the Midwest.