When Rachel makes a promise to Sarah, it comes with a hefty price. Meanwhile, Helena and Gracie work with the Hendrixes, and Donnie takes Kellerman to task. Over at Castor, Coady and Benchman scheme to track down the Castor Original, and Rudy descends on DYAD for a much needed asset.

Castor and Topside force Sarah to make uneasy alliances. But there is some rest for the wicked, when Donnie gives Helena an unexpected ‘gift’, and the results of Alison’s candidacy finally come in. Even as team Leda takes a moment to breathe, change is afoot and a life hangs in the balance.

After evading a Neolution attack in Iceland, Sarah returns home to track down an elusive new ally. As Cosima struggles with Delphine’s disappearance, Alison and Donnie host a pregnant and ever-hungry Helena. Meanwhile Felix, feeling like an outsider, embarks on his own journey of self-discovery.

As Sarah reels from the shocking discovery of Neolution’s biotech, she’s desperate to find answers. Rachel receives a visit from her keeper and takes a fraught step forward towards recovery. Meanwhile, Helena is mistaken for “Mrs. Hendrix” when she answers a visit from suspicious visitors.

Sarah realizes her goals have aligned with her once enemy and forms a precarious alliance in hopes of getting to the head of Neolution. Meanwhile, on her end, Alison dispatches Felix and Donnie to infiltrate a Neolution fertility clinic that Beth investigated. Back at the lab, Cosima and Scott receive a revolting delivery from Alison. As Rachel grows closer to Charlotte, Susan Duncan leaves a devastating decision in her hands.

Sarah’s day off to reconnect with Kira quickly turns into a sleuthing mission aimed squarely at discrediting Felix’s new ties. Cosima steps up with Donnie to further investigate the Neolutionist fertility research center. While Donnie bumps into an all too familiar face, Cosima makes a shocking discovery that leads her into a dangerous encounter.

When Cosima admits doubts about finding a cure in time to save her life, Sarah makes a lone decision that exposes Kendall to those she’s spent years hiding from. Meanwhile, Art and Felix team up to protect naïve clone, Krystal Godderitch, from herself.

Sarah regresses to old habits while struggling to cope with the consequences of her decisions —reconnecting with Beth on a dark night of the soul. Back at the lab, Cosima, haunted by vicious memories and news of Delphine, attempts a dangerous Hail Mary to find a cure. At the Hendrixes, Alison is more determined than ever to create normalcy for her kids. As she forges ahead with a birthday party, Donnie is caught in the Neolution crosshairs.

As Alison is terrified by Neolution threats made against Donnie, Sarah and Mrs. S question her loyalty to her sisters. While Sarah is distracted, Siobhan sneaks out to seek vengeance. Meanwhile, Cosima and Susan Duncan work together in a final attempt to discover a cure.

With Susan Duncan deposed, Sarah finds herself in the same position as Rachel — under the heel of a new HBIC — causing the archenemies to come together for the greater good. Cosima and Susan butt heads on the ethical implications of human experiments. Meanwhile, when the Hendrixes' legal counsel, 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.

When communication with Cosima is cut off and Neolution appears to be within striking distance, Sarah's alliance with Rachel goes out the window. Sarah lures a high-level Neolutionist into a trap, and sets out to bring down Neolution once and for all.

Following up on intel from a mysterious source, Mrs. S heads on the road with Sarah to investigate a lead on a Neolution defector. Meanwhile, an unexpected reunion takes place on the island, while Cosima's research leads her to a discovery more terrible than she could have ever imagined. As more pieces slide into place, the puzzle only seems to become more complicated.

While Sarah and Kira spend some well-needed bonding time at home, secrets are unlocked about Kira’s special gifts that are surprisingly linked to the island’s creature. Meanwhile, Cosima joins Delphine at the mansion for a twisted family dinner, causing a dangerous showdown with the Neolution patriarch.

Cosima is forced into an alliance when an old enemy returns to the island, accelerating P. T. Westmorland's sinister agenda. Meanwhile, on the mainland, Krystal stumbles onto a vital clue to the Neolution endgame. Sarah and Mrs. S’s research into Neolution's corporate ties leads to the cosmetics industry and an inevitable cross with Krystal Goderitch. But before Sarah can dig deeper, Krystal hijacks the investigation.