[Edit]
In the beginning it starts as a flashback where we learn about Teresa of the Faint Smile and how the Organization she works for operates. About how a very young girl tries to befriend her. This goes on for a short while then the story jumps into the worlds present time. And that young girl is now a woman named Claire and is the lowest ranked member of the Organization's Claymores. Follow along as Claire makes a similar friend as she was to Teresa and how she joins with fellow higher ranking Claymore to fight ever stronger "Youma" (demons). She and others will uncover more secrets that the Organization has kept hidden and have to make choices based on that.