From the book jacket:

The puckish super-being called Q has bedeviled Captain Jean-Luc Picard and the crew of the Starship Enterprise since their first encounter at Farpoint Station. But little was known of Q's enigmatic past or that of the transcendent plane where he sometimes dwells. Now Picard must discover Q's secrets for the sake of all that exists.

While the Enterprise struggles to survive an alien onslaught, Captain Picard has been kidnapped by Q and taken on an astounding journey back through time to that immeasurably distant moment when the Continuum faced its greatest threat. But far more is at stake than simply the mysteries of the past, for an ancient menace is stirring once again, endangering the future of the galaxy, and neither Q nor Starfleet may be able to stop it!