In this prequel to the great graphic classic, The Sandman, you learn how Morpheus, Dream of the Endless, ended up so weak and tired that he was able to be captured by mere humans. If you haven't read the original work, then really, go do that. Now. It's well worth your time. It will also help you to understand the magnitude of what is revealed in this.
If you have read the original work, then you will certainly recognize Morpheus' arrogance (which even he notices when faced with multiple aspects of himself), understand why it matters that he speaks to Delight, be shocked when there is something Destiny doesn't expect, and appreciate the revelation that for all the family squabbles of the Endless siblings, they have nothing on their parents.
In addition, as is usual in Gaiman's work, spun throughout the grand themes of immortals and their terrible mistakes, there are also small stories of individuals and their personal tragedies and triumphs.
Overture is a gorgeous prequel to a gorgeous work, and is well worth savoring.