From MangaHelpers:
In the month since she transferred to a new school, Misao hasn't made any friends. Instead, she's been teased and bullied by some of the more aggressive girls in class. This all changes on the day Sae returns to school. Sae is good at everything, and everyone at school adores her--but Sae seems to want only to be friends with Misao. All the bullying stops immediately, and Misao is thrilled to be best friends with the most popular girl in school. Sadly, things really are too good to be true. Sae starts to become more and more domineering, preventing Misao from making other friends and insisting on controlling how she spends all her time. As Sae's friendship becomes more and more nightmarish, someone reaches out to Misao to try to help her break free. But is it already too late?