51 Birch Street is a 2005 documentary film about the universal themes of love, marriage, fidelity, and the mystery of a suburban family.

A few months after his mother's sudden death from pneumonia, Doug Block's 83-year-old father, Mike, calls him to announce that he's moving to Florida to live with "Kitty", his secretary from 40 years before.

When Mike and Kitty marry and sell the Block family home, Doug returns to suburban Long Island for one last visit.

Among the mementos being packed away, Doug discovers three large boxes filled with his mother's daily diaries going back 35 years in which she recorded her unhappiness, her rage against her husband, her sexual fantasies about her therapist, a brief affair with an unnamed friend of her husband—and her suspicions about Kitty.

Through conversations with family members and friends and surprising diary revelations, Doug finally comes to peace with his parents who are more complex and troubled than he ever imagined.