Just after she graduated from college the future star won her first professional role. The play was due to open in Baltimore three days later. Now, on the way home to Connecticut with her parents, she finally told her father the news. For the whole four-hour journey he remained stonily silent and once they arrived home Dr Hepburn could contain himself no longer. Acting was "a sordid lifestyle, one step up from the streets". If she persisted, he'd cut her off. "Then do it, " she replied.

He slapped her across the face.

The 18-year-old didn't flinch or cry.

She simply turned around, walked upstairs and started packing.

Whatever happened she would be in Baltimore in three days' time.

According to William J Mann, in his comprehensive new biography Kate: The Woman Who Was Hepburn, that is emblematic of Kate Hepburn. At a time, he contends, when film studios created and honed their stars in the same way that Detroit turned out Ford T motor cars, Hepburn created herself, created her own lifestyle, acting style and image.

Moreover, if the public failed to react with enthusiasm, or sensed an awkward truth lurking behind the projection, Hepburn would reconstruct her image until a facet caught the limelight once more. Is such an untarnished attitude towards a screen goddess from Hollywood's golden age fair? And, even more importantly, is such an unglamorous explanation of her stardom realistic? On both counts, yes; but there are other facets to Kate Hepburn's appeal that Mann also brings to light.

For a woman who came from such a respectable backgound, Hepburn had a surprising amount to conceal from the press and public when she became a film star in 1934. Firstly, there was the family. From today's point of view, Hepburn's parents would bring kudos to anybody aspiring to be in the public eye. From a 1930s perspective of lowbrow, antisexual American society, however, her father's specialisation in sexual deseases and, even worse, her mother's activism for birth control threatened to abort Hepburn's new-found fame. Then, there was her sexuality.

Despite devoting fully 50 pages to the subject, Mann still pussyfoots around Hepburn's sexual orientation. Was she straight or bi? Butch or femme?

Mann can't make up his mind, although from all his evidence it seems obvious: Hepburn wore pants, liked being affectionate with slim, boyish girls, socialised with gay men because they weren't sexually threatening and lived with women all her life.

By contrast, Mann exposes Hepburn's famous long-term affair with the actor Spencer Tracy as a publicrelations scam, a convenience for both - particularly for the homosexual Tracy, who consoled himself for his apparent affliction with repeated returns to the bottle.

Curiously, at the outset of her career, Hepburn did not try to hide her sexuality.

During the making of her first movie, she laughed when she received the news that her current girlfriend, the heiress Laura Harding, had responded to the bald question, "Who are you?" from a RKO executive with:

"Miss Hepburn's husband."

Also, references in the press to constant "lady companions" would not have mattered, if Hepburn had not purposly chosen for her third movie to be an exploration of a female cross-dresser who attracts both men and women. Nowadays, Sylvia Scarlett is regarded as one of camp cinema's overlooked gems. It was a public-relations disaster for Hepburn. Critics accused her of being an "oddity", a "freak of nature" who was a "better looking boy than girl". Hepburn got the message. Off-screen she would continue to live, albeit more discreetly, with Harding and successive women. On-screen, however, her alter ego would learn the error of her ways.

A Philadelphia Story is a frothy, champagne comedy, yet Hepburn is chastised in the film not only by her uncle and father but also by both her ex- and would-be husbands. The first image we see is Hepburn being knocked to the ground and a door being slammed in her face. She's a punchbag, a flighty aristocrat brought to earth by a succession of manly men.

Even so, the play and subsequent film were both concocted in close alliance with Hepburn. "Make her like me, " she told the playright, Philip Barry, "but make her go all soft." The public lapped it up. And, again, 12 years later when her career was threatened by offscreen events and theatre exhibitors claimed she was "box office poison", Mann emphasises that Hepburn repaired the damage by changing her image on the screen.

Brought up by her parents in the east-coast Emersonian tradition of bringing morality to politics, Hepburn was undoubtedly sincere in her defence of Communist colleagues when they were blacklisted by the studios during the McCarthy era. Unlike other liberals like Humphrey Bogart or Ronald Reagan, she never recanted or backed down in her support. Still, she knew enough about public perception to recognise that she had to at least appear to bend to the political wind. So she sought the role of the gutsy, patriotic spinster in The African Queen and, despite playing a Brit, became established as "an American exemplar".

Those two films defined the stardom of the complicated and often contradictory Hepburn.

Mann's book shows that, no matter how manipulative her behaviour, Hepburn was also loyal and always generous. Yet there is one story about Hepburn which calls for pause. One night on Broadway in 1938, the star returned to her dressing room to find a burglar rifling through her jewel-case.

Hepburn sent him screaming down 42nd Street with the yell, "Just what the hell are you doing?"

After confronting all the selfdetermination in this thoroughly engrossing book, one can all too easily understand the robber's reaction.