"If you want to ever wear clothes again, listen to what we have to say."
In TAG, a strange fortune teller has begun a horrific game using young college girls as her pieces. Hypnotizing them and enforcing strange rules in the back of their minds, the girls are eventually approached by a naked woman in a public place. Quickly kissed by the girl (called being "Tagged" by the people playing the game), they find their bodies will no longer let them wear clothing.
Shortly thereafter, they find instructions and an explanation. If they ever wish to wear clothing again, they have to find another girl playing the game, and Tag her with a kiss. Until then, their hypnotism won't allow them to cover themselves in any way.
Mizumo, the main character, observes someone playing the game but is unable to catch her in time to question her. Shortly thereafter, she visits a strange fortune teller...