As Clarke tries to provide Madi a new start, she and the other Wonkru survivors face a tall order in rebuilding Sanctum with the Prime worshipers, the Children of Gabriel, and the former Eligius prisoners. Echo, Hope, and Gabriel try to rescue Bellamy before an unseen foe drags him into the anomaly.

Hope returns home and remembers her childhood with Diyoza and Octavia -- including Octavia's choice to abandon them and return through the anomaly 10 years ago. In the present, Gabriel and Echo discover that the others on this planet have a connection to Earth.

Ravel faces a deadly threat when Sanctum's power reactor threatens to melt down. Clarke struggles to keep the peace on the eve of Russell's execution, unaware that an even more dangerous foe now controls his body.

Hope, Echo, and Gabriel devise a long-term strategy to escape from the prison planet where Hope has grown up. Meanwhile, a group of mysterious outsiders arrive on Sanctum with knowledge of Clarke and her people.

Octavia finds herself a prisoner on Bardo, a planet settled by advanced humans who subject her to interrogation. The Prime worshipers begin a gruesome self-immolation to demand Russell's release, and Indra discovers a wolf in the fold.

Clarke and her friends explore a seemingly desolate ice planet in search of a way to rescue the others. Diyoza escapes her confinement, and Sheidheda makes his next move -- forcing Indra into a difficult decision.

While Murphy has a face-to-face with the Dark Commander, Emori tries to heal old wounds by reuniting the Children of Gabriel with their families. The prisoners on Bardo must reckon with their past, while the local leader hopes to recruit them to his cause in the coming war.

Clarke comes face to face with Bill, the Shepherd, who reveals the fate of the planet Earth, how he met Becca Pramheda, and the nuclear apocalypse that ended the world more than 100 years earlier.

Diyoza, Hope, Echo, and Octavia are trained on Bardo, where Anders attempts to convince them to give their hearts and minds to the Cause. Murphy and Indra try to defuse a tense situation after Nikki and the Children of Gabriel take hostages.

Clarke and the others attempt to rescue Octavia, Echo, and Diyoza from Bardo, only to learn that their friends might not want to be rescued. Now on the loose inside Sanctum, Sheidheda tries to convince the old tribes to follow him as the Commander.

Bellamy Blake makes his way across an unforgiving land in an attempt to reach another portal, along with a fellow survivor from Bardo -- who is a faithful follower of the Shepherd.

Clarke, Echo, and Raven find themselves at odds with Bellamy when they learn of his allegiance to the Shepherd, who is searching for the Flame in hopes of triggering the Last War. Back in Sanctum, Indra, John, and Emori try to save lives from Sheidheda's oppressive rule.

Malachi's dark forces prove little match for the Shepherd when he brings a contingent of his disciples to Sanctum in search of the Flame. But the arrival of the red sun complicates everyone's plans.

After the group find themselves back on Earth Clarke destroys their means of returning to Sanctum. But while the survivors of Earth get accustomed to their new reality, the Shepherd sends a dangerous hunter after Madi.

In the aftermath of an explosion inside the bunker on Earth, Clarke struggles with Madi's fateful choice -- while John, Raven, and Jackson try desperately to save Emori.

Cadogan embarks on a journey to meet the people who created the stones, believing he can initiate the ascension of the human race. While Indra and Raven lead Wonkru against the Disciples on Bardo, Clarke takes matters into her own hands.