Siff, a zealous student in sexology, has recently moved to her boyfriend Lucas' place. Her close friend Maria is temporarily sleeping on their couch, though Lucas is not very keen on the idea. At first it seems the two women keep the phlegmatic Lucas well in hand. But when Siff is gone for a weekend seminar, Lucas has an affaire with Maria. On finding out what has happened Siff wants to throw Maria out, but she doesn't have the heart to do it, so Lucas ends up promising Siff it will never happen again. But it does, Lucas and Maria still feel attracted to each other. Neither of them wants to break it up. As a solution the two women decide to share and sleep with Lucas by turns. But Siff knows from her studies that a threesome relationship will inevitably go on the rocks without a set of clean-cut agreements. So they work out a sex-calendar to manage their ménage à trois. Realizing she is trapped by her own broad-mindedness Siff understands she needs to set a limit.