This week's flashback shows something the Zhat Vash call the Admonition Commodore Oh oversees it as Narissa and her aunt, Ramdha, undergo the ritual with others. Touching an artifact left behind by a dead civilization shows what that society saw before its end, a kind of hell brought on by the creation of sufficiently-advanced synthetic life. Having survived the ritual, they become part of the Zhat Vash, which was formed after the Romulans first discovered the Admonition. Assimilating Ramdha's memories of the Admonition is what caused the Artifact to fail and disconnect from the Borg hivemind.
In the present day, Narissa is still hunting for Elnor aboard the cube. He gets an assist from Seven, who decides they should try to reclaim the cube. She hooks herself into the cube, temporarily becoming a Borg Queen and forming a new hivemind for the drones left onboard. She's too late. Narissa jettisons the remaining drones into space before taking off with her Romulan fleet to find Soji's home planet. The cube is left to Seven and the ex-Borgs.
On the La Sirena, Rios is stunned when he sees Soji. He locks himself in his quarters. It takes some time, but Raffi manages to piece together some of what happened to Rios by questioning the various holograms on the ship. She gets the rest of Rios himself. Rios' old ship encountered emissaries from Soji's homeworld, including one who looked just like her. His captain received orders from Commodore Oh to kill both emissaries, otherwise, their ship would be destroyed. The captain followed his order and Rios couldn't forgive him for it. The captain couldn't live with the guilt and killed himself.
Picard learns from Jurati what happened to Bruce Maddox. Jurati explains that Oh showed her what will happen if the synths are allowed to flourish, that they'll something someone or something that will bring hell to the galaxy. Picard is understanding, but Jurati agrees to turn herself in when they reach the nearest starship.
All of this new information along with her old research gives Raffi what she needs to put together what happened with the attack on Mars. The Romulans discovered the Admonition and made it their purpose in the galaxy to prevent it from happening again at any cost. When Dr. Noonien Singh begins creating androids as advanced as Data, they send Oh -- half-Romulan, half-Vulcan -- to infiltrate Starfleet. From that position, Oh orchestrated the attack on Mars, forcing the ban. Having halted the production of new synthetic life, all that's left is to eradicate those who are left from the galaxy, which is why they've been searching for Soji and her homeworld. Now they're headed there to wipe the synthetic race out.
Admiral Clancy agrees to send a squadron to aid Picard in stopping the Zhat Vash attack, but Soji can't wait. She plots a course through the Borg transwarp network back to her homeworld. Rios object, but Picard convinces him to go forward with it. They plot a curse and head to Soji's home.