Nick Vecchio is a clinical psychologist who lives in Portland, with his wife Emma, an English professor, and his college-age son and daughter. One morning in early April a young colleague, Ralph Hafer, stops by his office—briefly and without notice. Ralph is deeply troubled over having uncovered an incident of serious malpractice involving a "fragile patient" and he needs his former mentor's advice. He offers few details except to say that the situation is abusive and he is uncertain how to respond. They schedule a time to meet, but the next morning Ralph is stabbed to death in his office at the DeLong Institute, where Nick was once the chief psychologist. Bernal Walker, a paranoid schizophrenic and one of Ralph's patients, is apprehended immediately. Despite Ralph's death and without the name of either the patient or therapist, but urged on by Ralph's widow and ethically obliged to prevent harm, Nick decides to exercise his status as an Oregon Psychology Board member to investigate Ralph's allegations, a decision that for the next few weeks will catch him and his family up in both Ralph's murder and his legacy.
Nick meets with Lieutenant Hollis McDougall, the chief investigator into Ralph's murder, and asks for his help to access DeLong Institute records. The detective reluctantly agrees, and then asks Nick's opinion: Why would Walker's blood analysis show high levels of speed (Dexedrine) and no trace of his prescribed anti-psychotic tranquilizer (Triavil)? As if to seal their unorthodox partnership. they share candies from the Mint Humbug tin that Nick keeps in his desk.
Nicks inquiries lead him to the DeLong Institute, where he uncovers more wrongdoing, and to Evingston College, where he learns that Ralph was soon to join the faculty. He begins to wonder if Ralph's murder might be connected somehow to his malpractice allegations, and several times visits the boarding house where Bernal Walker lived. He gets to know the aged owner-manager, Adele Athens, and one of the disabled residents, Cliff Sprando. At police headquarters, assisted by his son, Andy, Nick reviews Walker's clinical file. He finds no evidence that Walker is Ralph's "fragile patient" but is baffled by the fact the Walker was never prescribed Dexedrine, was not known to use street drugs, and has no documented history of violent aggression. he then visits the state mental hospital where Walker is housed, only to come upon a grisly suicide.
McDougall is about to close his murder investigation when Nick discovers a startling link between Dexedrine and Triavil. He suspects that Walker himself was a victim: manipulated with drugs and suggestion to kill Ralph. McDougall is skeptical, until a series of incidents suggests that Nick is being stalked. Emma insists that he stop playing amateur detective.
Still searching for Ralph's "fragile patient" Nick visits an AIDS hospice where he finds critical evidence regarding Ralph's malpractice allegations. The next morning at predawn, alone in his office, he is confronted by the abusive therapist. The malpractice scenario is fully revealed but the encounter turns ugly. Nick is kidnapped and injured, then rescued by two unlikely champions. He knows now that Ralph's murder has nothing to do with Ralph having discovered the abusive malpractice.
Later that day, still groggy from pain medication, Nick makes a series of phone calls that reveals a strange set of coincidences involving Mint Humbug candy. He now knows that Ralph's murder was not a simple case of "mental patient goes crazy," but was accomplished via the expert exploitation of a vulnerable person. He...
