一周内，王勇结识了校园乖乖女小薇，又于某夜在兼职酒吧里认识了小薇的孪生姐姐Mona。姐妹俩长相、身材都极为相似，性格却截然不同且互称“不和”。王勇白天与小薇约会，夜晚又被Mona投怀送抱。

　　慢慢地，王勇感觉怪怪的。姐妹俩的发质、肉感以及身上的气味，都完全是同一人。而朋友小张亦发现Mona情绪很不稳定，有暴力倾向。更让人怀疑的是，姐妹俩从未同时出现过。就在此时，小张被Mona勾引，以性交易为条件被其控制。而王勇只顾追问小薇关于她姐姐Mona的事。

Fate has split the lives of two beautiful twin sisters. While Yung falls for the gentle and beautiful Siu Mei, his buddy Cheung dates Mona, Siu Mei's violent twin sister. As Siu Mei and Mona have never appeared at the same time, Yung suspects something odd about them. Later, Cheung is found brutally murdered.