Such is the bare plot of one of Japan's biggest pop-culture sensations - a manga story with a staggering 27 million copies in print over 13 volumes, making it as much a sociological as a publishing phenomenon.

The Nana series has spawned novelettes, foreign translations, CDs, a looming US release and a movie, which will screen at the Asian Hong Kong Film festival, which opens this week.

Shibuya, Tokyo's mecca to teen fashion, is a regular haunt for youngsters sporting the Nana look - punky or demure, depending on which character they identify with.

Even in a country boasting millions of adult comic-readers and a manga market worth more than US$4.48 billion, with plots dealing with everything from corporate battles to suicide, the success of Nana has stunned the publishing world.

'We've never seen anything like it,' says a spokeswoman for publisher Shueisha, which has just released the much-awaited Volume 13. She says she has no idea why the comic has struck such a chord, but Nana's author, Ai Yazawa, recently gave a clue in an interview with a magazine.

She wrote Nana because she wanted to try to help women make it through their difficult 20s. 'Realising that you're not alone with your pain and self-doubt can be a source of comfort,' Yazawa said.

Japan's best-selling female manga artist said she saw her two characters - the romantic Komatsu and the tough, but insecure, Osaki - as examples of the same modern woman.

Not that Yazawa is touting social realism, or even feminism. Like most characters in the shojo (girl's comics) genre, the Nanas are baby-eyed caricatures more concerned with clothes, make- up and the mysteries of the XY chromosome than attacking the citadels of male power. The magazine that serialises Nana, Cookie, is a shop window for beauty products.

Still, the two Nanas experience traumas beyond the limits of typical shojo themes. For much of the early series, Komatsu is torn between the affections of two men before becoming pregnant and deciding to get married. Her friend must resist pressure from her lover to have a child so she can pursue her career. Sex, contraception and deadbeat boyfriends provide constant bitter grit for what might otherwise be a sweet confection.

There have been hit manga in Japan before - Yoshinori Kobayashi has become a multi-millionaire since the late 1990s by writing revisionist comics arguing that Japan should stop apologising for its role in the second world war and be more assertive. But if Kobayashi has tapped into Japan's anxieties about its post-cold war role, Yazawa has tapped into women's concerns about their role in a changing Japan.

Japanese women in their 20s are rebelling in unprecedented numbers against the traditional narrow roles assigned to them, working longer and putting off marriage, often until it's too late to have children.

The fertility rate this year dropped to a postwar low of 1.29 per woman, one of the lowest in the developed world, sparking some desperate initiatives by candidates in the looming general election to persuade women to have more babies.

Like the drama played out in Nana, many women are torn between a childless career and life at home with an overworked salary-man. In their 20s, there's still space to fantasise about putting this choice off forever, finding the sort of companionship they crave in a female friend, instead. Indeed, Nana hints at something deeper: one of the more controversial scenes in the new movie is a kiss between the two leads.

As the success of the story has grown, the real-life dramas of the two Nanas have been swamped by increasingly strained plot machinations. Readers left Volume 12 with Osaki poised on the brink of stardom and pursued by paparazzi, even as her friend considers marriage. What will happen in 13? Eventually, Nana will end, leaving millions of fans still searching for answers.

Hong Kong Asian Film Festival, Thurs-Oct 11, $50, $65. For programme inquiries, go to www2.cinema.com.hk