Barbara Covett, the keeper of an acerbic diary, is a domineering and solitary teacher who rules with an iron fist over her classroom at a decaying state-run secondary school in London. Save for her cat, Portia, Barbara lives alone, without friends or confidantes--but her world changes when she meets the school's new attractive art teacher, Sheba Hart. Sheba appears to be the kindred spirit and loyal friend Barbara has always been seeking. But when she discovers that Sheba is having an incendiary affair with one of her young students, their budding relationship takes an ominous turn. Now, as Barbara threatens to expose Sheba's terrible secret to both her husband and the world, Barbara's own secrets and dark obsessions come tumbling to the fore, exposing the deceptions at the core of each of the women's lives.