This was a sequel to an episode of Little Girl Lost (1973) with Mike Connors, Pernell Roberts, Julie Adams and Beverly Garland all reprising their roles. The grown-up daughter was played by a different actress (Susan Lee Hoffman) rather than Dawn Lyn, who, as a teenager, stopped growing, and had trouble getting adult parts, and retired from acting.
The title refers to "hard-boiled" which is a classical description of a private detective who has seen lots of trouble (crimes, death, domestic problems, et cetera) and he no longer outwardly reacts to it.
Julie Adams (Edie Reynolds Fallon) and Beverly Garland (Stella Carter) were both born on October 17, 1926.
Mike Connors reprises his role as Mannix. He did it again in 2003 in a comedy called Nobody knows anyhing!