Alliances are shattered between Sarah and Rachel as Cosima is cut off from communication and the Neolution moves in. Later, Sarah sets a trap for a high-ranking Neolutionist in hopes of bring it down for good.
With Evie Cho in charge of Neolution, Sarah finds herself in the same position as Rachel causing the archenemies to come together for the greater good. Cosima and Susan strongly disagree on the ethical implications of human experiments. When Adele further investigates Donnie’s case, she begins to suspect Felix is keeping deep secrets. Helena’s arrival forces Felix to make a difficult decision.moreless
Sarah and Mrs. S question Allison's loyalty when she becomes terrified by Neolution threats made against Donnie. With Sarah distracted, Siobhan sneaks out to seek vengeance. Cosima and Susan Duncan work together in one last attempt to discover a cure.
Sarah goes back to old habits to cope with her decisions and Cosima attempts a Hail Mary back at the lab.
Cosima admits doubt about finding a cure, which causes Sarah to make a drastic decision that exposes Kendall to people she has been trying to hide from. Elsewhere, Art and Felix try to protect Krystal Goderitch from herself.
Sarah has a sleuthing mission to discredit Felix's new ties after she tries reconnecting with Kira. Also, Cosima teams up with Donnie to further investigate the Neolutionist fertility research center.
Sarah forms an alliance with a former enemy in hopes of getting to the head of Neolution. Elsewhere: Alison orders Felix and Donnie to invade a Neolution fertility clinic and Susan Duncan leaves Rachel to make a devastating decision.
Sarah has a traumatic encounter as she investigates a shocking discovery of Neolution's biotech.
Sarah tracks down an elusive new ally and unearths the next aspect of the Neolution's agenda.
In the fourth season premiere, a Neolution attack destroys Sarah's refuge in Iceland and forces her to flee once again.