Naoko Noda, who works in a trading company, is called "anego" by her younger co-workers.  What is "anego?"  It means "older sister", but in this case it is given as a nickname to Naoko because she is like a big sister to the other workers, who depend on her to give them advice about everything.  This drama, based on a popular novel, depicts realistically how Naoko lives her everyday life, including her love life, the problems she faces, and her uneasiness about the future.

Naoko, 32, works in the corporate strategy division.  She is single, and has been working hard for 10 years, now she often finds herself in the role of training and guiding the younger female workers.  Recently she realizes that most of the temporary and contracted workers' goals are to find an elite male employee within the company to marry.

One day, Naoko goes out to play golf with the general manager and other co-workers, and there she unexpectedly meets her former co-worker, Eriko Sawaki.  Eriko seems to have a picture-perfect life; she is married to a company president, and already has one child.  Naoko gets distracted by Eriko and the happy atmosphere around her, and ends up with a terrible golf score.  After finishing the round, Eriko's husband comes to pick her up, and Naoko is surprised when she sees his face.  Eriko's husband is the handsome man who saved her from the drunk man.  Naoko's heart starts pounding, and she can’t help but envy Eriko's happy life.

在一流商社工作届满10年的30代单身熟女OL（白领丽人）野田奈央子（筱原凉子饰），后辈们都叫她「大姐头」，是其他女职员倾诉烦恼的对象。现在主要做一些年轻女职员的指导工作。在公司内年轻的派遣女职员和签约女职员们的目的多半是与优秀的男性职员结婚成为了一种风气。奈央子也抱着一份终有一天符合她条件的真命天子出现在自己身边的理想理论，但终究抓住优秀男性职员心的还是那些年轻的派遣女职员。

某一天应同事邀请，与部长们一起去打高尔夫球，在那里偶然遇见比奈央子小两岁的以前公司的后辈沢木绘里子。与公司的社长结婚并且育有一女的绘里子简直就是理想中的「胜者组」。被绘里子的幸福感而失控的奈央子以悲惨杆数结束了高尔夫。高尔夫结束后，看到来接绘里子的丈夫，奈央子很是吃惊。叫做沢木翔一的绘里子的丈夫竟然是奈央子在电车里遭醉汉纠缠时，出面解围的人。「大姐头」的主人公是在都市一流商社工作的32岁的单身女性。在职场上已没有她的前辈，只剩后辈们叫她「大姐头！」。可是，飒飒风姿之下，却有着对未来的不安和焦躁，心底深处不为人知的脆弱。通过真实的抓住主人公的烦恼和恋爱，描写了即使这样也毅然面对人生拼命努力的形象。是一部充满了30代女性的恋爱与人生真实面的电视剧。