Happy Face is inspired by the true-life story of Melissa G. Moore; the critically acclaimed Happy Face podcast from iHeartPodcasts and Moore; and the autobiography Shattered Silence, written by Moore with M. Bridget Cook. At 15, Moore discovered that her beloved father was the prolific serial killer known as Happy Face. As an adult, she has changed her name and guarded her secret while her father has been serving life in prison.

Jumping off from Moore's true-life story, the Paramount+ Original Series follows Melissa and her incarcerated father, known as the Happy Face Killer. After decades of no contact, he finally finds a way to force himself back into his daughter's life. In a race against the clock, Melissa must find out if an innocent man is going to be put to death for a crime her father committed. Throughout, she discovers the impact her father had on his victims' families and must face a reckoning of her own identity.