The series follows a pair of troubled teenage girls, and their encounters with a dangerous supernatural force that may or may not be the answer to their problems.
After Evie Grayson witnesses a murder committed by drug dealers, she is targeted by local crime lord Sebastian Polk. She discovers a mysterious ally in the forest, the titular Birch — a fierce guardian that will destroy her enemies when summoned. But as gruesome deaths plague the small town, Evie begins to question the nature of her supernatural protector.
In a parallel story, Lanie Bouchard is the adopted daughter of a local politician and has just given birth in secret. Her parents insist the baby is her "sister", and go to increasing extremes to keep Lanie away from her child. Troubled by strange visions and a power she doesn't fully comprehend, Lanie fights to save herself and her daughter from the Bouchard's control.
The Birch provides examples of:
- Abusive Parents: The Bouchards are extremely controlling of Lanie, drugging her and locking her inside the house to prevent her from acting out.
- Bait-and-Switch: It appears at first that both segments chronicle the Birch protecting two people, Evie and Lanie. But then it's revealed that Lanie isn't protected by the Birch, she is the Birch, having originally been a human capable of controlling plantlife and eventually transformed into the titular creature when her powers skyrocketed in a bout of grief.
- Children Are Innocent: Gavin Polk is a typical 10-year old boy, seemingly unaware of his family's criminal empire. Thurston actively works to keep it that way.
- Infant Immortality: Averted. Lanie's newborn daughter Laura is poisoned by the Bouchards with nightshade.
- Jerk with a Heart of Gold: Thurston Polk works for his uncle selling drugs, and is the initial antagonist of the story. But his POV reveals that he tries to spare Evie out of guilt for her mother's death, and is working on a plan to escape from his abusive uncle.
- Meanwhile, in the Future : The series jumps between the stories of Evie and Lanie, one in modern times and one decades prior, who both have a connection to the Birch.
- Missing Mom: Evie's mother died of a drug overdose prior to the series. The Polks provided it, creating the initial conflict between the Polks and the Grayson family.
- My Secret Pregnancy: Lanie's parents took her out of school for "Mono", and kept her at home until she gave birth. Afterwards, they actively try to convince her that the new baby is her "sister".
- Sympathetic P.O.V.: Thurston comes across as a dangerous thug initially, but the episodes from his POV often show him in a much better light.
- Tragic Monster: Lanie becomes the titular Birch, consumed by her rage and grief at the death of her daughter.
- Would Hurt a Child:
- The Birch captures Gavin and buries him in the garden, though it isn't clear whether she would have killed him or not. Evie has considerable difficulty getting him back, once she realizes her mistake.
- Sebastian Polk, on the other hand, has absolutely no issue with hurting his nephew. One of his goons threatens to slice up Gavin's face on his orders, and when Gavin defies him to save Thurston, Sebastian slashes the child with a power saw and kicks him several times.