At age sixteen, Lisa Shilling has a serious mental illness. She has good days, when she is bright and cheerful, but she also has dark days, which occur with increasing frequency, when her behavior is inexplicably erratic. Lisa hears voices, and sometimes speaks suddenly and rudely with a clipped British accent. Her changes in mood and personality come without warning, and are terrifying both to herself and those around her. Knowing that something is very wrong, Lisa tells her parents directly that she is ill and pleads for help, but, preoccupied with their own affairs, they are in denial, more concerned with appearances than the well-being of their daughter. Unable to get the intervention that might save her, Lisa's behavior grows increasingly destructive and bizarre.

Acknowledging what the adults in their world cannot, Lisa's friends band together to help their troubled classmate. Mary Nell, or M.N., is the studious one who pores over psychology books, trying to figure out what is wrong with Lisa and what can be done to help her. Betsy Goodman, the narrator of the story, is practical and straightforward, better able to connect at times with Lisa on a personal level than the overly-analytical M.N. Elizabeth Frazer, chosen as a confidante of sorts by Lisa herself, has, unbeknownst to the others, been through a similarly frightening experience, and so is able to approach the situation with a greater level of understanding. M.N., Betsy, and Elizabeth are aware that they are woefully underqualified to know how to handle Lisa's deteriorating mental state, but by offering their support and availability to assist in any way they can, they manage to enable Lisa to stay afloat until she can get the professional help she so desperately needs.

First published in 1969, Lisa, Bright and Dark was one of the most widely read of the genre which includes other popular psychological novels such as Joanne Greenburg's I Never Promised You a Rose Garden and Beatrice Mathews Sparks's Go Ask Alice. Admittedly, developments in the study of mental health over the past forty years have left much of the information in the book outdated, but Lisa, Bright and Dark still enjoys a substantial readership, with its latest republication date in 2007. Because of its sensitive portrayal of the uncertainty and stigma associated with mental illness, the book maintains a relevance that resonates to the present day.