Jeannie Ferrami, Psy.D., is an associate professor and criminality researcher at the fictional Jones Falls University, an Ivy League school in Baltimore, Maryland. She studies the influence of genetics on personality. Her interest in criminal tendencies is influenced by the fact that her father, Pete, is an incarcerated burglar. Financially strained, she sends her Alzheimer's-afflicted mother to live in a sub-par nursing home. Jeannie's friend Lisa Hoxton is raped during the evacuation of an on-campus locker room. The police determine that the perpetrator was a serial rapist who simulated a fire. Lisa works with sympathetic police Lt. Michelle Delaware to create a facial composite of the suspect. Jeannie meets law studentSteven Logan, who participates in her study, prompting mutual attraction. Jeannie's software finds links in medical data and has identified him as the twin of incarcerated murderer Dennis Pinker. This seems to confirm Steve's fears that he is unable to control his own violent impulses. Berrington "Berry" Jones, a prominent researcher at JFU, is shocked to see Steve. He contacts his two partners in Genetico, Inc., a medical research company that heavily funds JFU; Jones, Preston Barck, and United States Senator Jim Proust are racist and classist, and apparently believe that the involvement of Steve and Pinker in the study will jeopardize Genetico's $180 million sale to international conglomerate Landsmann, and with it Proust's presidential campaign. Berry disrupts Jeannie's research by alerting the press to the legitimate ethical issues of her software. Soon after, Steve is arrested for Lisa's rape and Lisa picks him out of a lineup, but Jeannie believes his claims of innocence. Steve is released on bail. Jeannie and Lisa visit Pinker; he is identical to Steve, but had no twin at birth and was born two weeks after Steve, in a different state. Both men's fathers were in the military when the couples sought fertility treatments at the Aventine Clinic in Philadelphia, Pennsylvania. She visits Aventine, which was founded by Genetico in 1972 to pioneer in vitro fertilization. Jeannie escapes an attack by the rapist, whom she mistakes for Steve. Returning to Baltimore, she learns Steve was there during the attack, confirming that the rapist was the titular "third twin." The identical men are clones, illegally implanted into women treated at Aventine. Jeannie's software is run on an FBI database, but Berry forces the ethics issue; to access the search results, Jeannie must prevail in a discipline-committee hearing. Steve competently defends her, but Berry secures Jeannie's dismissal via bribes. However, Pete has been released from prison for good behavior; he helps Jeannie steal the data. Jeannie travels to New York City with Lt. Delaware to meet search result Wayne Stattner. He is a fourth twin; although it's apparent Stattner is a sadist, he has an alibi for Lisa's rape. Steve's father, Col. Charles Logan, reveals that the motivation for the cloning was a "super-soldier" program; Steve was literally bred to be a killer. Col. Logan runs Jeannie's search on The Pentagon's computers, yielding three suspects: Henry King, George Dassault, and Harvey Jones. Jeannie and Lisa prove that Harvey Jones of Philadelphia is the rapist. Harvey is Berry's son, whom Berry sends to spy on Jeannie by impersonating Steve. Jeannie detects him and Steve subdues him. Jeannie, Lisa, and Steve crash the Landsmann-Genetico press conference. Steve tries to spy on Berry, who restrains Steve and frees Harvey. Lisa brings King, Dassault, and Stattner to the press conference; Harvey is present and Steve also arrives, drawing the press' attention. Steve realizes that his identity is determined by free will, and not by his genes or upbringing. Nine months later, Jeannie and Steve prepare to take their honeymoon. Genetico and its founders have been discredited, Pete has started a profitable private security business, and Harvey is in prison. Jeannie has taken a lucrative position at Landsmann, and has moved her mother into a better facility.
Film
A 1997 television film based on the book starred Kelly McGillis as Doctor Jean Ferrami and Jason Gedrick as Steve Logan, Harvey Jones, and the other clones.
