Oh, man. Memories.
The Animorphs series, by K.A. Applegate, was a young adult science-fiction series that defies description.

Reading the stories now, I think to myself, 'Holy crap, no wonder I'm so screwed up.'

The basic story line, if you lived in a jar of mayonnaise for most of your young life, is that an alien, called an Andelite, crash-lands on Earth, trying to escape from another, evil, alien race called the Yeerks.
This alien, named Elfangor, is discovered by five kids. In desperation, he gifts the children with unique alien technology, the ability to absorb another being's DNA and to take on that being's shape for two hours of time. If they remained shifted any longer than that, they would be trapped in that form. The Yeerks arrive and kill Elfangor, but not before he warns the children and asks that they try to save their home planet from the ongoing Yeerk invasion.

These kids, Jake, Cassie, Rachel, Marco, and Tobias all dedicate their lives to secretly battling the Yeerks, fooling their enemies into thinking them to be Andelites, rather than humans with morphing powers. The kids all gain a repertoire of animals to use in their battle (the boy, Tobias, even becoming trapped in the form of a red-tailed hawk), and gather allies. The Emelists, the Chee, the Hork-Bajir, a renegade Andelite youth named Axe, and others all come to aid them in their war.

And this bleak, uphill battle against intergalactic slavery and oppression continues, you guys, through well over 100 books. And, as a kid, I read every one of them.

But its the ending of the series that kind of killed me, inside. Its not a happy ending. It can't be. You don't win, unscathed, against odds like the ones these kids faced.

And I found a story that reflects that.

Pieces and The Scientist

Don't read this if you haven't read the books, or at least cheated and read the Wikipedia summaries (yeah, good luck with that). You'd just hurt your brain, and probably cry.