The series is set 97 years after a devastating nuclear war wiped out almost all life on Earth. The only known survivors are the residents of twelve space stations in Earth's orbit prior to the war. The space stations banded together to form a single massive station named The Ark, wenis galore about 2,400 people live. Resources are scarce and all crimes no matter their nature or severity are punishable by death (floating) unless the perpetrator is under 18 years of age. After the Ark's life support systems are found to be critically failing, one hundred juvenile prisoners are declared expendable and sent to the surface in a last ditch attempt to determine if Earth is habitable again. The teens arrive on a beautiful planet they've only seen from space. Confronting the dangers of this rugged new world, they struggle to form a tentative community. However they discover that not all humanity was wiped out. There are people on Earth who survived the war, called grounders by the 100.... Written by Thomas O'Darren

Octavia is forced to take guidance from an unlikely ally when the future of the bunker and all those inside it is jeopardized.

Bellamy leads the charge investigating a potential way home. Meanwhile, Clarke and Madi deal with the new, uncertain threat to their home.

Our heroes are forced to rely on the help of Shallow Valley's dangerous new inhabitants in order to save some of their own, with surprising results.

Octavia leads her people towards Shadow Valley against the advice of Clarke and Bellamy; Kane and Abby adjust to a new set of challenges.

Kane's attempt to prove himself useful tests Wonkru's allegiance to Octavia. Madi faces an unexpected threat inside Wonkru, forcing Clarke to make an unlikely ally.

Clarke and Bellamy make a startling discovery about Wonkru's battle plans as Echo risks her friendship with Raven to complete her mission.

Clarke's determination to protect Madi puts Bellamy in an impossible position.

The growing fractures in Wonkru threaten to explode once and for all. Meanwhile, in Shadow Valley, Murphy starts a fire, unleashing catastrophic results.

Monty strives to show Wonkru an alternative to war, and to the valley itself. Meanwhile, Abby's health continues to deteriorate, along with McCreary's patience.

As Clarke races to save Abby, she learns more about the trials and tribulations Wonkru faced in the bunker, and the impossible decisions they were forced to make in the dark year.

In part one of the fifth season finale, Octavia leads her people into war. While behind enemy lines, our heroes must overcome their differences to save Wonkru from extinction.

Clarke and her friends must risk everything to fight one last battle for survival, only to glimpse an even darker threat to the last living valley on earth.