Sasuke from the previous life lost Sakura on the 5th great war, but he made a promise to her that the next time that he meets her he'll never letting her go.
With Naruto gone on his training journey with Jiraiya, Sakura finds that she's falling deeper and deeper into darkness without his cheerful presence. So she tries to figure out how he was able to see such a beautiful world, when all she saw is an ugly one