When Barbara Covett (Dame Judi Dench), a domineering and solitary teacher at a decaying state-run secondary school in London meets the school's new art teacher, Sheba Hart (Cate Blanchette) her world is turned upside down. 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 (Bill Nighy) and the world, Barbara's own secrets and dark obsessions tumble to the fore, exposing the deceptions at the core of each of the women's lives.