霍金斯一家是一个充满爱和困扰的家庭。妻子经常出差不在家，家里的孩子不打扫房间使家里很脏乱。所以霍金斯决定买一个合成人来处理这些家务。他们给这个合成人取了名字叫安妮塔。女主人劳拉出差回家后隐隐觉得安妮塔不太对劲，和其他合成人不太一样，安妮塔会在晚上一个人看着月亮发呆，这对于合成人来说事很不正常的行为。随即劳拉开始观察安妮塔的一举一动，处处提防着她。

女主人劳拉越来越感到受到了合成人安妮塔的威胁，孩子们对合成人安妮塔的依赖远远大于劳拉，小女儿每晚要求安妮塔给她讲睡前故事而不让劳拉讲；儿子也越来越在意安妮塔的一举一动，甚至在这个家里有一种完全不需要劳拉存在的感觉，这让劳拉很烦躁。妓院里，妮斯卡再也忍受不了，她打伤了男顾客，并自行拔出了芯片，从妓院里逃了出来。

安妮塔在交通事故中救了劳拉的儿子托比，乔和托比都很感谢安妮塔，并为她处理“伤口”。这让劳拉很不好做，所以不得不和家里的孩子们一起支持安妮塔，即使现在她心中还存在着许多疑惑。劳拉还是没有放弃对安妮塔的观察，只是不再做的那么明显，表面上一片祥和的氛围。乔治博士正计划着与他的初代合成人Odi一起逃出医用合成人的监视，他把医用合成人锁到了阁楼里准备出逃，然而医用合成人火爆的脾气让这个计划中途失败。与此同时乔治博士也意外的将Odi丢失在了森林里。

劳拉从一些细节里感觉出来安妮塔真的存在问题，所以她通过很久的游说终于说服了丈夫要带安妮塔去做个检测，结果检测的结果证实的劳拉的猜测，安妮塔果真和其他合成人不一样，她已经15岁了，而一般合成人最多使用五到六年，这个消息让乔很惊讶。男主人乔想要把安妮塔送回厂家处理，他认为既然已经过了试用期的合成人留在家里就是隐患。但劳拉和两个女儿还有儿子都不同意。在劳拉和丈夫乔争吵的时候，大女儿Mattie开车带走了安妮塔并决定去找莱昂。 失落的莱昂命令麦克斯不要跟着他，而他自己去了湖边。伤病导致莱昂晕倒在了湖边，麦克斯带着Mattie去寻找莱昂。尼丝卡看到反合成人的组织在仓库里肆意殴打、破坏合成人，尼丝卡便大打出手打了这些人。

尼丝卡打人的视频流传出来之后，“合成人杀手在我们周围”的新闻迅速被传开，政府一面隐藏这个消息一面派合成人研究人员去抓住尼丝卡，但公众对合成人的恐慌还在日益增加。而另一方面警察凯伦也在四处寻找尼丝卡中，可是凯伦寻找尼丝卡的目的好像和其他的警察并不一样。利奥用尽方法想唤醒在安妮塔的深层结构中米娅的记忆。但是一直失败，他不得不绝望的承认米娅确实已经不再这个世界上了。

弗莱克醒来发现自己在研究员的家中。带他到郊外告诉他利奥发现他们大脑中隐藏的秘密，并告诉他自己的上司希望毁灭他但是自己是他的朋友。弗莱克打死看守抢到手枪，抢夺汽车逃走。劳拉和丈夫乔、大女儿Mattie的误会越来越深，在女儿Mattie的逼迫下，劳拉终于说出了自己的心里事情。原来劳拉的母亲因为童年劳拉的弟弟去世的事情一直怨恨着劳拉并且不肯原谅她。所以劳拉一直认为自己处理不好和女儿的关系，她怕失去女儿。利奥通过米娅和玛丽蒂的提示终于找回了米娅的记忆。这时逃出的弗莱克打电话给利奥，约在父亲大卫明信片中的地点见面。

莱昂得知尼丝卡被警察和合成人研究人员追查，让尼丝卡躲到安全的地方，这个地方就是乔治博士的家里。 乔治博士收留了尼丝卡，尼丝卡为了不让医用机器人发现他的身份就以人类的身份生活在乔治博士家中。乔治博士给她讲了过去很多关于制作她们这批特殊的合成人的事情。与此同时凯伦找到了尼丝卡，原来凯伦也是有意识的合成人，而她被制作出来以后就因为和莱昂的母亲长得一样所以被抛弃了。她希望尼丝卡和她自己一同消失在这个人类的世界中，并用抢威胁尼丝卡，却无意间开枪打死了乔治博士。博士临死前让尼丝卡逃走，尼丝卡联系了莱昂，莱昂让尼丝卡躲到劳拉家中。

麦克斯在被合成人研究人员追铺的时候为了救莱昂掉进了河里，所以芯片收到了损伤。莱昂找到他后用尽了办法也只能处于开机状态而恢复不了意识。莱昂决定用父亲藏在这六个特殊的合成人记忆中的内容来修复麦克斯。劳拉一家被合成人研究人员控制在家中，但他们依旧在想办法救这些朋友，她们用能证明合成人有意识的视频要挟合成人研究人员放了莱昂和安妮塔她们。凯伦的目的一直是让有意识的合成人全部消失，但是在莱昂给她打电话之后心软了，她在最后关头同意和其他五个人一同运用记忆中的内容救麦克斯，而在救完之后就选择了离开，依旧一个人生活。

The Hawkins family try to get their lives back on track. Niska is driven to a fateful decision while on the run in Berlin. Leo, Max and Mia are living in hiding near a coastal town when they are threatened by a mysterious organization.

Laura must decide whether to accept Niska's request, while Karen and Pete learn about a special Synth being sold for unprecedented amounts on the black market. Hester uses questionable means to gain information from her prisoner, and Leo faces a new mission. Mia takes a risk to help Ed but is her secret now in jeopardy?

Athena travels to the UK and her tragic secret is revealed. Niska's consciousness assessments begin. With more time at home Joe notices Sophie's increasingly eccentric behaviour. As Karen returns to work with Pete, cracks start appearing in their relationship. The Synths are forced to leave their hideout but Mia shocks them with a huge decision.

Mia and Ed get closer but their relationship is thrown into jeopardy when their secret is discovered. Toby gets closer to Renie but offends her when he breaks through her synthie mask. Things look bleak for Niska until Laura brings in a surprise witness. When Milo confronts Athena about her secret agenda, she persuades him her research could be mutually beneficial. Karen and Pete delve deeper into the mysterious 'seraphim' investigation. As Hester's influence over Leo becomes clear, cracks appear between him and Max.

When Mia risks falling into the hands of a mysterious organisation, she is forced to take drastic action. Odi struggles to find meaning, while Sophie is awestruck by Renie's 'Synthie' lifestyle. As Pete recovers from his assault, Karen makes an admission that rocks their relationship. Hester and Leo stake out the Silo and track down the sniper who killed Ten. Hester agrees to deal with her peacefully, but can she be trusted? Mattie leaves home to reconnect with Leo.

Athena challenges Milo's ethics and is shocked when he reveals his real plan. Mia's surprise return has ramifications for the Hawkins family. Mattie finds Leo but can she trust him with her secret? Max resolves to begin a new community of Synths; when he learns how far Leo is prepared to go to complete his mission, he takes matters into his own hands.

When Mattie reveals that the Synths are planning to break into the Silo, Laura is determined to find Niska and ask for her help. Mattie asks Max to help save Leo from Hester's influence. Toby receives a disturbing message from Renie. Karen seeks out Athena's expertise and makes a life-altering decision. Mia and Hester break into the Silo, but things don't go according to plan.

The acclaimed drama concludes. In the aftermath of the Silo tragedy Karen feels she and Sam have no future and takes matters into her own hands. Niska and Astrid prepare to return to Berlin, but can Niska really walk away from her family? Joe leaves Laura shell-shocked with a frank admission. Hester seeks revenge and will stop at nothing to hurt Leo.