After Octavia is taken, Bellamy is not far behind her. This turns into Gabriel, Echo, and Hope trying to track down where he was taken while also adjusting to working together. Hope can't remember anything but she does have a note that tells her to trust Bellamy.
The three of them track down those invisible people, yet they arrive at the Anomaly entrance. So Echo sees hallucinations of Roan and of the real Echo, which causes her to kill three of them. They figure out they need to travel into the Anomaly though and end up going in together.
Changes in Sanctum mean different factions are working against one another. Clarke, Indra, and the others need to find a way to have everyone work together to build a compound and find peace.
But with Russell still alive, and begging for death, Clarke has to deal with him first. She isn't really reacting to the fact that he killed her mother, in fact she spends most of the time not admitting it to herself.
Then when he hands her clothes that Simone left behind of Abby's, Clarke snaps. She starts to beat up Russell, which means he gets knocked out and Sheidheda kills him to take over his body.
Clarke doesn't realize that and she lets the Sanctum castle burn, before announcing that she will be executing Russell for his crimes.