Pretty Little Liars: Original Sin takes viewers to Millwood, a blue-collar town still healing from tragic events that took place 20 years ago. An unknown assailant has begun tormenting a disparate group of teen girls - a new generation of "Little Liars" - to make them pay for their parents' secret sin... as well as their own.

After receiving a series of increasingly disturbing texts, Imogen and her newly-formed friend group - Tabby, Faran, Noa, and Mouse - plot to end Karen's reign of terror. Later, Imogen finds herself in an unexpected position just in time for the Spirit Week dance, while Karen and her twin sister Kelly hatch a payback plan of their own.

In the wake of Spirit Week's deadly aftermath, Imogen attempts to connect the dots between her former BFF, her deceased mother, and the mysterious "A." Then, as Sheriff Beasley closes in, the fate of the friend group hangs in the balance.

With her mother's house now on the market, Imogen consults the supernatural for answers. After being forced to spend time with Kelly, Faran notices a string of peculiar coincidences - and enlists Henry for help testing her hypothesis. While shooting her latest short film, Tabby remembers a harrowing trauma, and Noa comes face-to- face with a terrifying foe.

Unwilling to part with her mother's house, Imogen throws an epic Halloween rager to raise some cash to save it - and enlists her friends to help. At the party, Faran puts her Kelly theory to the test, while Mouse wades in dark waters tied to a mysterious core wound. Later, Tabby confronts Tyler and Greg for their toxic behavior.

After bonding over shared trauma, Imogen and Tabby take a road trip to Rosewood, where they double down on their efforts to uncover the truth about past events. Meanwhile, Noa attempts to help her mom, Mouse goes on her first date, and, armed with a tip from "A, Faran works to heal old wounds.