BOOK REVIEW : Upgrades Enliven Stock Characters : AN AWFULLY BIG ADVENTURE <i> by Beryl Bainbridge</i> , Harper Collins $19.95, 193 pages

Stella Bradshaw, the teen-age heroine of the wickedly diverting novel “An Awfully Big Adventure,” is ungrateful, rebellious and willful--traits that exasperate the well-meaning aunt and uncle who have adopted her.

In an attempt to channel her wilder impulses, they send her to elocution class, but instead of taming her vowels so she can rise above her humble Liverpool origins, the classes simply fan her already flaming spirit. When Stella decides to audition for a place in the struggling local repertory theater, neither Aunt Lily nor Uncle Vernon objects.

They’ve done their best, and if Stella refuses to learn shorthand and rejects the security of a job in Woolworth’s, then the stage is better than the streets, if only marginally. Although most of the author’s attention is lavished upon the rep troupe, the character of the doting, rejected Uncle Vernon is a triumph: a total immersion course in the English class system and how it operates.

Scruffy and barely solvent, staffed by a collection of second-raters, has-beens and never-will-bes, the theater is still a wonderland to an adolescent who grew up in the repressive atmosphere of a provincial boardinghouse.

In the group of assorted eccentrics who make up the company, the personality traits that so perplexed Stella’s aunt and uncle become positive assets. Although she was such an indifferent student that her teacher actually urged her to leave school, Stella applies herself so conscientiously to her new duties that she’s almost immediately promoted from gofer to bit player.

The credit for her astonishing transformation belongs to the director, Meredith Potter. Precociously wise in many ways, Stella is hopelessly naive in others, falling hopelessly in love with Meredith without ever suspecting that his natural inclinations totally preclude any reciprocation. Meredith accepts her slavish devotion as his artistic due, apparently quite unaware of its true source because his proclivities are so obvious to everyone else. Although in other circumstances this situation would be both shopworn and implausible, author Beryl Bainbridge focuses so acutely on her young heroine that the reader sympathizes with Stella’s bewilderment.

To her, Meredith’s flamboyant mannerisms merely represent the obvious differences between a theatrical genius and lesser mortals. Because no one in the company bothers to enlighten her, she must interpret the clues for herself, hampered by total unworldliness and dazed by love.

The author has wisely set her story in an era when the age of innocence lasted far longer than it does today, a point drolly emphasized when one of the senior actresses presents the 17-year-old Stella with her first bra.

Poignant, arresting and often funny, an accumulation of such small, telling details effectively recreates the atmosphere of Liverpool in the mid-’50s: a gritty, exhausted city pocked by bomb damage, only just beginning to emerge from the anguish of the war, still a decade away from the hectic vitality that enlivened England in the 1960s.

Spurned by Meredith and eager for sexual experience, Stella cheerfully submits to the amorous advances of the once-celebrated actor who fills in for the disabled star of the local troupe. Tutored by this gentle roue , our Stella shows an aptitude for love equal to her gifts for drama.

Succinct and tart, “An Awfully Big Adventure” never takes itself too seriously, the ironic intent underlined by a title suggesting a bedtime story for grown-ups. Gleefully exploiting the limits of her material, Bainbridge manages, against all the odds, to recycle stock characters and situations into a sophisticated entertainment.

Next: Carolyn See reviews “Object Lessons” by Anna Quindlen (Random House).