This is a story of what you will do to save the life of the one you love, or, what love is capable of. It has got to be one of the best STTNG books I've ever read (see my review of Time's Enemy to see the BEST ST book of all time), and it is the only book I have ever read that shows, once and for all, what real love is all about.
The book opens with a completely demoralized, elderly Admiral Riker grumpily running Starbase 86, with a nervous lieutenant looking after him. The lieutenant tells him there is a message from Betazed: Luaxana Troi is dying, she wants him to come to Betazed. He gets there, and she doesn't die until she impresses upon him that it's his fault that her daughter, Deanna, is dead. That's okay, Admiral Riker's life has deteriorated because he believes it, too.
Then Data tells him that it may not be true, that in another timeline, Deanna lives. He explains that in this timeline, not only did she live, but she stopped a peace treaty with the Sindareen, a warrior race that is similar to the Ferengi in that they will trade with anyone...but their trade is based on terrorizing worlds and stealing everything they sell.
When Riker hears the story of how Deanna didn't die, he immediately sets out to go back in time and save her from her fate. Data decides to stop him at all costs, in keeping with the Temporal Prime Directive.
Riker cannot live without trying to prevent Deanna's death, and Data cannot live with allowing Riker to stop it.
Not to give away too much of the ending, BUT in the end, Riker turns out to be right and Data is wrong.
This is true love, fellow readers. That's all that this book is about. I cannot stress it enough: this is the story of how one person can change your life permanently, and without them, you are lost. It's about the lengths one will go to to save the one they love, and, in the end, how love can be the strength you draw from when you have nothing left.