In flashbacks we see Octavia's time on Bardo where she meets Levitt who tries to see her memories. She resists until he spots Hope in them and then agrees if he leaves Hope out of it. They get to the end of Season 3 before Hope arrives after Dev died and takes Octavia back to Sanctum, seconds after she disappeared after Diyoza.
Hope tries to find her mother but gets captured, so she agrees to tag Octavia and then Octavia and Bellamy get taken. Octavia again sees Levitt who tells her how to resist the machine, but right after Anders announces that Bellamy is there and she needs to talk him down.
In present day Hope, Echo, and Gabriel get to Bardo. Echo and Hope keep killing people who get in their way, but they are able to locate Anders and hear more about the history of Bardo. They then find Octavia who has been there a week and who is resisting their memory capture. Echo forces the person extracting her memories to show what happened to Bellamy.
He shows her a memory where Bellamy took everyone hostage for Octavia, but she tries to strike a deal with Anders so that Bellamy goes home while she tells Anders everything. Even about Clarke. Anders sets the Anomaly stone somewhere but then a Bardo disciple sets off a bomb. Bellamy is nowhere to be found, so the assumption is that he died.
Echo is so angry that she kills the man and risks everyone's safety and Hope's ability to find Diyoza.
Meanwhile in Sanctum the faithful want Russell and threaten to kill themselves one by one if they don't get him. Murphy tries to stop it but they realize he isn't one of them. "Russell" comes in to save the day but proves to Indra that he is Sheidheda. She doesn't tell anyone but she has Nelson and Jackson take out his memory drive so when he dies he won't be able to come back.