In "Hart to Hart: Secrets of the Hart," Jonathan and Jennifer Hart are living their luxurious life in San Francisco when Jonathan receives a phone call from his long lost sister, Susannah. She tells him that she's in town with her son, Joshua, and wishes to meet them. Jonathan is excited about the possibility of reconnecting with his only blood relative, and Jennifer is just as enthusiastic about welcoming them into their lives.
When they finally meet, Susannah reveals that she wants to sell her family home, which has been closed for years, and needs Jonathan's help to locate some key documents related to the property. As Jonathan and Jennifer begin to search for the papers, they realize that Susannah and Joshua may not be as innocent as they seem.
As they begin to unravel the family's past, they discover that Susannah's husband was involved in criminal activity, and the documents are crucial to a criminal investigation. The family may be in danger, and Jonathan and Jennifer must act fast to protect them.
Meanwhile, the Harts' investigation into Susannah's past leads them to interact with shady characters, including an opportunistic real estate developer. They also discover long-hidden family secrets that change Jonathan's understanding of his past.
In the end, the Harts help Susannah and Joshua to reconnect with their estranged relatives, but not before narrowly escaping death multiple times. The family learns that their bond is much stronger than they thought, and Jonathan finally finds the closure he was seeking from his past.