Upon Strawberry Hill lay three boarding schools for girls. Men are forbidden to set foot on this land, so most of the students are lesbians. Tamao is...well it's technically not correct to call her a stalker because it's implied that stalkers keep their distance and watch/voyeur from afar. Tamao 'stalks' her roommate, Nagisa (main character: a transfer student) from up close and doesn't care about being inconspicuous. The first thing Tamao did when Nagisa came to school was to take her to a room and bring out a tape measure, recording every detail, all while Nagisa was conscious and squirming. When Nagisa's uniform arrived, Tamao insisted on putting it on for her - starting with unbuttoning her pajamas. She gets as far as sleeping in the same bed as Nagisa.
Despite the huge list of 'offences' (read: teasing/pseudo-stalking), Tamao genuinely does care for Nagisa. When Nagisa needs to do work, e.g. cram for exams, Tamao is respectful and switches to 'supportive wife' mode. In times of sorrow, Tamao is there to hold her and let her cry. Why is she so obsessed with Nagisa? Nagisa being delicious to tease isn't the only reason...