ينشأ "غاس" ويترعرع على يد والده داخل كوخ في إحدى الغابات النائية، ويتلقّى دروسًا عن البقاء ومخاطر العالم الخارجي الكامنة وراء السياج.

يتتبّع "غاس" رجلًا ضخمًا متردّدًا في رحلته إلى المجهول، ويكتشف د. "سينغ" تكلفة إنقاذ "راني"، فيما تجد "إيمي" بداية جديدة بعدما ينتهي العالم.

يتلهّف "الضخم" لمضيّ "غاس" في طريقه، فيتوصّل إلى طريقة ليخبّئه على مرأًى من الجميع، بينما يحضر آل "سينغ" حفلة في الحيّ تأخذ منعطفًا مشؤومًا.

يصبح "غاس" تحت حماية أصدقاء جدد، ويتوصّل إلى حقيقة ماضي "الضخم"، ويخشى د. "سينغ" أن تكتشف جارته الفضولية سرّ "راني".

يصطحب "غاس" و"الضخم" رفيقة سفر تقترح عليهما طريقًا مختصرًا محفوفًا بالمخاطر، ويتعرّض ملاذ "إيمي" للتهديد، ويوشك وقت آل "سينغ" أن ينفد.

في لحظة يأس، يقطع الطبيب "سينغ" وعدًا جريئًا، ويتسبّب تذكار مفقود في رحلة وعرة إلى "كولورادو"، ويضيّق "الرجال الأواخر" الخناق على "المحمية".

تتسبّب رحلة "غاس" للعثور على "بيردي" واكتشاف المزيد عن صلتها بوالده في جعله متشككًا في كلّ شيء قد عرفه يومًا.

يتزلزل عالم "غاس" فيهرب ويتّخذ قرارًا متسرّعًا، ويبدأ "الضخم" في استيعاب أخطاء مضى عليها زمن، وتخوض "إيمي" مواجهة أخيرة مع الجنرال "أبوت".

Locked up in the zoo's kennels, Gus meets his fellow hybrid prisoners as he looks for a way out. General Abbot pressures Dr. Singh to produce a cure.

An uneasy alliance forms over plans to free Gus and the other kids from captivity. Desperate for a breakthrough, Dr. Singh conducts a risky experiment.

Gus' expedition with Dr. Singh leads to several shocking discoveries. Big Man and Aimee seek refuge and reinforcements. Becky connects with a new friend.

As Big Man reckons with sins of the past, Gus becomes a target of anger and suspicion, Rani's misgivings multiply, and Becky begins basic training.

General Abbot makes an example out of Gus. Dr. Singh obsesses over his research ahead of an important presentation. Becky crosses paths with a frenemy.

The zoo turns into a battle zone when the rescue mission takes flight, with families coming together — and falling apart — during the chaos.

After an eventful, obstacle-strewn journey to find a safe haven, Gus learns more about his origins and how far Birdie was willing to go to protect him.

Warned about General Abbot's plans, Gus and his friends prepare to take a stand and defend the hybrid kids, no matter what it takes.

Gus and his cohorts set out for Alaska to reconnect with Birdie, who's working on a cure for the Sick. But first, they test their luck at an old casino.

After a doctor joins Gus and his crew, they seek a way to the coast and stumble upon a plane owned by a family in desperate need of medical attention.

As Gus learns more about a threat and their dangerous plans, the group races to catch a departing boat. Jepperd and Becky face an impossible decision.

An unsettling discovery at sea forces Gus to take on a grim chore as tensions swell between his companions. On land, an enemy interrogates Becky.

As polar night looms, Gus and his crew arrive at the outpost — but they're not alone. To find Birdie and the cave, they must brave several obstacles.

After a daring rescue, Gus meets an individual well-versed in hybrid history. Amid emotional reunions and eerie revelations, all roads lead to the cave.

With loved ones beside him, Gus goes on a treacherous journey into the cave to find a cure once and for all. Meanwhile, chaos breaks out at the outpost.

After suffering an unimaginable loss, Gus must make a choice that will determine the fate of humans and hybrids. Who will be left to tell their story?