破镜重圆 (pò jìng chóng yuán)
In the Northern and Southern Dynasties (nán běi cháo 南北朝,420-589AD) when the State of Chen (chén guó 陈国) was facing its demise, Xu Deyan (xú dé yán 徐德言), husband of the princess, broke a bronze mirror into halves. Each of them kept a half as tokens in case they were separated. Soon afterwards, they did lose touch with each other, but the two halves of the mirror enabled them to be reunited. This idiom is used to refer to the reunion of a couple after they lose touch or break up.