From the book jacket:

The mischievous creature who calls himself Q has subjected Captain Jean-Luc Picard and the crew of the Starship Enterprise to many of their strangest experiences. But little had been known of Q's curious existence or that of the advanced dimension from which he comes. But now Picard knows more than he ever dreamed about an ancient conflict whose consequences might spell the doom of the entire galaxy.

The galactic barrier has fallen and Q's oldest enemy is free once more. Captain Picard and his crew find themselves in the middle of a cosmic war between vastly powerful entities. The future of the Federation may be at stake, but how can mere mortals turn the tide in such a superhuman battle? Picard has to find a way, or neither the Q Continuum nor the galaxy will survive.