While staking out a warehouse robbery, Pete accidentally injures a young woman named Billie. Guilt-ridden, he falls in love with Billie, an aspiring folk singer with a hauntingly beautiful voice. Things become unexpectedly complicated when Billie's brother, Case, is one of the robbery suspects. Case learns that Pete is an undercover cop and, desperate for money, demands that Billie get the goods on Pete to lead them to a big robbery.
Linc receives a desperate call from an old friend, now an inmate at a mental institution. Shortly afterward, the friend is dead of "heart failure", only the latest of a number of strange deaths at the place. Linc goes undercover as a patient at the institution, hoping to find out what's going on.
A hard-nosed ex-cop returns from Vietnam to investigate the death of his brother, who was killed in a car accident just after he called him and told him that he had seen a murder, and that the murderer was an undercover cop.
The trio becomes involved in the case of a young boy who's being raised in an orphanage. His mother is not taking care of him, but she does not want to give him up for adoption, and she is involved with a drug dealer.
Pete is knocked out and jailed in a small town when he tries to assist Levi Frazee, an Indian who is believed to be withholding the whereabouts of his cousin, who is suspected of murder. Later the main witness, the victim's wife, changes her story and claims it was Levi who killed her husband.
Linc comes to the airport to meet a friend who is returning from Vietnam, only to learn that he was killed by a sniper on the day he was supposed to go home. Suspicious of the story of how his friend died, he wonders is any of the four other servicemen in his unit who have returned know more than they are telling.
Julie is staying at a beach house where she is helped by a kind old fisherman who lives nearby. He tells her to ring her porch bell whenever she needs his help. When she visits his house the next day, someone else lives in the house. She can't find the mysterious old fisherman except when she rings the bell, he suddenly shows up. Is he for real or a figment of Julie's imagination?
Student activists protesting for academic changes at their college take over the administrative building, and hold Julie and an elderly and ill female teacher hostage. Meanwhile, a seemingly quiet but demented student named Walter has taken their otherwise peaceful fight into fanatical-warlike territory with a rifle and other weapons! It's up to both the men of Mod Squad and the hostages to stop Walter before he kills any more innocent people.