Ringer follows Bridget Kelly, a woman who has been sober for six months and is a witness to a mob murder. Despite the protection of the FBI, she fears for her life and flees to New York to reconnect with her estranged twin sister, Siobhan Martin. However, when Siobhan disappears under mysterious circumstances, Bridget assumes her sister's wealthy, seemingly perfect life, only to discover that Siobhan's life is just as complicated and dangerous as the one she left behind. As Bridget navigates the high-stakes world of her sister's marriage to Andrew Martin and her affair with her best friend's husband, she realizes that someone is actively trying to kill Siobhan. Forced to maintain the charade to survive, Bridget must uncover the truth about her sister's past while staying one step ahead of the hitmen, the FBI, and the secrets that threaten to expose her true identity.