Frank Galvin (Paul Newman) was once a promising graduate of Boston College Law School and a lawyer at an elite Boston law firm. But he was framed for jury tampering some years back by the firm's senior partner because he was going to expose their corrupt practices. The firm fired him and his marriage ended in divorce. Although he retains his license to practice law, Frank has become an alcoholic ambulance chaser who has had only four cases over the last three years, all of which he has lost.
As a favor, his former teacher and friend Mickey (Jack Warden) sends him a medical malpractice case in which it is all but assured that the defense will settle for a large amount. The case involves a young woman who was given an anesthetic during childbirth, after which she choked on her own vomit and was deprived of oxygen. The young woman is now comatose and on a respirator. Her sister and brother-in-law are hoping for a monetary award in order to give her proper care. Frank assures them they have a strong case. Meanwhile, Frank, who is lonely, becomes romantically involved with Laura (Charlotte Rampling), a woman he meets at a local bar.
Frank visits the comatose woman and is deeply affected. He then meets with the bishop of the Archdiocese of Boston (Edward Binns), which owns the Catholic hospital where the incident took place. As expected, the bishop's representative offers a substantial amount of money   $210,000   to settle out of court, but Frank declines the offer as he fears that this may be his last chance to do something right as a lawyer, and that merely taking the handout would render him "lost". Everyone, including the presiding judge and the victim's relatives, is stunned by Frank's decision (Frank fails to communicate the offer to his client's family before rejecting it).
Things quickly go wrong for Frank: his client's brother-in-law finds out from "the other side" that he has turned down the $210,000, and angrily confronts Frank; his star medical expert disappears; a hastily arranged substitute's credentials and testimony are called into serious question on the witness stand. His opponent, the high-priced attorney Ed Concannon (James Mason), has at his disposal a large legal team that is masterful with the press; the presiding judge (Milo O'Shea) makes deliberate efforts to obstruct Frank's questioning of his expert; and no one who was in the operating room is willing to testify that there was any negligence.
Frank's big break comes when he discovers that Kaitlin Costello (Lindsay Crouse), the nurse who admitted his client to the hospital, is now a preschool teacher in New York. Frank travels there to track her down, leaving Mickey and Laura working together in Frank's Boston office. Frank confronts Costello, asking, "Will you help me?"
Meanwhile in Boston, Mickey is looking for cigarettes in Laura's handbag and discovers a check from Concannon's law firm. He infers that she is a mole, providing information on their legal strategy to the opposing lawyers.
Mickey flies to New York to tell Frank that Laura has been betraying them. He suggests to Frank that it would be easy to get the case declared a mistrial, but Frank decides to continue. Shortly thereafter, Frank meets Laura, who has also traveled to New York. In a display of cold fury, Frank strikes her in the face, knocking her to the floor.
Costello testifies that, shortly after the patient had become comatose, the anesthesiologist (one of the two doctors on trial, along with the archdiocese of Boston) told her to change her notes on the admitting form to hide his fatal error. She had written down that the patient had had a full meal only one hour before being admitted. The doctor had failed to read the admitting notes. Thus, in ignorance, he gave her an anesthetic that should never be given to a patient with a full stomach. As a result, the patient vomited and choked.
Costello further testifies that, when the anesthesiologist realized his mistake, he met with Costello in private and forced her to change the number "1" to the number "9" on her admitting notes. But Costello made a photocopy of the notes before she made the change, which she brought with her to court. She was subsequently fired, leading her to exclaim in court, "Who are these men? I wanted to be a nurse!" But Concannon quickly turns the situation around by getting the judge to declare the nurse's testimony stricken from the record on technicalities. Feeling that his case is hopeless, Frank gives a brief but passionate closing argument, telling the jury "you are the law" and entreating them to seek "truth and justice" in their hearts before they vote.
In the penultimate scene, the jury   apparently disregarding the judge's instructions to ignore the nurse's testimony   announces that they have found in favor of Frank's clients. As Frank, Mickey, and Frank's clients quietly rejoice, the foreman asks the judge whether the jury can award more than the amount the plaintiffs sought. The judge resignedly replies that they can, but the amount they decide on is not revealed. As Frank is congratulated by his clients, Mickey, and colleagues and strangers alike, he catches a glimpse of Laura watching him across the atrium.
One night, Laura, in a drunken stupor on her bed, drops her whiskey on the floor, drags the phone toward her, and puts in a call to Frank. As the phone rings, Frank sits in his office with a cup of coffee. He moves to answer it, but ultimately decides not to. The film ends with the phone continuing to ring.
Output: Who becomes Frank's star witness?