Ah-jung is a married woman in her mid-30s and has a five year-old son. She ghostwrites other people's biographies and at the moment, she is writing the biography of "an insurance sales queen." When Ah-jung's husband goes away on a business trip, she has no choice but to let her mother stay in their apartment. Ah-jung has problems dealing with her mother, a meddlesome woman who throws constant fits, probably the result of never having been loved by her husband. Mother also has a habit of putting mayonnaise in her hair to make it shinier..

Struggling artist Min Hee-do is offered three billion won to bet his life to a game against a rich old man, Kang No-sik, who is dying from a terminal illness. The game is for each man to dial a random phone number and guess if the person who answers will be male or female. Hee-do loses the bet, and after a month long brain operation, he wakes up to find that they have swapped bodies.

永民（金宋 饰）虽然顽皮，但仍算懂事。他每天放学去卖冰激凌赚钱就是为了给患有眼疾的母亲买一副墨镜。最终母亲还是发现了他这个秘密，他忍着委屈的泪水，任由母亲责骂抽打。因为出身贫寒，他常受到同学们的欺负，经历得多让他的心智比同龄的孩子要成熟。 一个从首尔转校到永民班上的女孩，名叫雨莲（李世英 饰）这个清秀的女孩，更主动示好。但雨莲把永民写的情书叫给了老师，被老师责骂一番的永民毕竟是孩子，在桌子上画上三八线表示不再跟雨莲交往。一次，永民勇敢救起了失足落水的雨莲，也知道了雨莲背后的故事。自此，两个背景完全的孩子成为了好友。