Barnaby Jones
The second-season episode "Requiem for a Son" introduced the character of Barnaby Jones (Buddy Ebsen), an aging veteran private investigator who had retired and turned over his agency to his son, Hal. However, Hal is killed. With the aid of Cannon and Hal's widow, Betty Jones (Lee Meriwether), they hunt down Hal's killer. Afterward, Jones decides to come out of retirement. "Requiem for a Son" was planned as a second-season Cannon episode, but when Barnaby Jones was sold as a separate series, the script was reworked into the premiere of that show. William Conrad appeared as Cannon in the guise of a special guest star.
There was additional "crossover" between the series. The first part of the two-part episode, "The Deadly Conspiracy", was aired during a fifth-season Cannon episode on September 17, 1975; the second part was aired two nights later as Barnaby Jones's fourth-season premiere.
Read more about this topic: Cannon (TV series)