Over the Edge by Jonathan Kellerman is a tale of intrigue and serial killings among the Los Angeles gay community. The hero, Dr. Alex Delaware, a clinical psychologist, receives a call from a previous patient from five years ago, Jamey Cadmus, a gay youth of eighteen, who has since become schizophrenic. Jamey babbles incoherently while asking Dr. Delaware for help. Alex learns that the boy is in the Canyon Oaks Mental Hospital, and drives there to visit Jamey. After difficulties getting into the clinic, he finds Jamey has gone missing.

The next day Milo, Alex's best friend, a detective investigating a series of murders called the Lavender Slasher Murders, calls and asks about Jamey. Milo asks only if the teen has called him, so Alex says yes. Later, Souza, the Cadmus's family lawyer, informs Alex that Jamey has been found at the scene of a murder similar to the Slasher killings, whose victims were all young boy prostitutes. The case pits Alex and his gay friend Milo against each other as Alex tries to prove Jamey innocent and Milo attempts to convict him. Souza hires Alex to prove that Jamey is incompetent and not aware of his actions when committing the murders. Alex reluctantly agrees to try to find evidence that will convince him that Jamey's capacity to reason is diminished, even though he feels many so-called "dim cap" defenses are faked.

During his investigation, Alex grows suspicious of Souza, and is relieved when Souza fires him; however, he continues to investigate independently. Learning from Milo that some biker thugs may be involved, Alex follows that lead. Investigating further, he questions a number of Jamey's fellow teens. He also researches schizophrenia and about drugs that mimic schizophrenia. The story follows Alex's investigations around the streets of Los Angeles, where he finds a trail of poison, wealth, greed, betrayal, and shattered friendships.

Eventually Alex learns that Jamey's step-aunt, Heather, resents Jamey and has been slowly poisoning him over the previous year with a drug that induces schizophrenia. He learns, too, that Heather has forged her husband's signature to get Jamey declared incompetent and thus gain control of the Cadmus family's assets. He also finds that Souza has conspired with Heather and has had his chauffeur commit the series of murders that implicate Jamey.

Souza has conspired with Heather because he envies his old friend, Jamey's deceased grandfather, Jack for having possessed many things Souza desired. For example, Jack had married his old girlfriend, Antoinette, and had been more successful in business as well. Jack had felt sorry for Souza, and even gave him land in an attempt to retain his friendship. Nonetheless, Souza pursues his hatred by punishing Jack's offspring through two generations by appearing to be a friend, but eventually almost destroying Jamey with the help of the equally bitter Heather. Jamey's poisoning and the deaths of the victims in the Lavender Slasher Murders are the end result of greed, envy and hatred on the part of previous generations.