During the retrieval mission to capture itachi in order to get Sasuke, Sakura ends up fighting with Karin and getting infected with a virulent corrosive chakra developed by orochimaru. In order to save her life, Kakashi makes the rash decision to replace her eyes (the point of contamination) with those of Itachi's. As she heals and becomes used to her new sharingan (with the grudging help from Sasuke), Sakura starts to see snippets of Itachi's life as see through his eyes. It's through these visions /memories that certain truths are revealed and she has to decide what to about the information she's learned.