Section 20 assesses the repercussions of Leo Kamali's execution. Sgts. Michael Stonebridge and Damien Scott fear that al-Zuhari will come after Kamali's daughter Ester for retribution. They head to the girl's school to bring her to safety, but her "Uncle" Qassem has gotten there first. Ester is confused about which one to trust, and flees into the crowd when Scott breaks the news that her father is dead. Qassem and the boys both follow in pursuit, escalating into a gunfight. Ultimately, Ester is safely relocated to the crib.
Locke interrogates Dr. Valigny, a prisoner from the bio-medical facility, who gives up the sponsor of the smallpox research -- German businessman Maroun Saleh. Martinez and Richmond track Saleh to a fortress where he's accompanied by Reza Hassan, al-Zuhari's number two. Scott and Stonebridge infiltrate the stronghold in a raid that results in the deaths of both Hassan and Saleh.
Richmond analyzes the intel Stonebridge extracted from the fortress. The team learns that Andersson uploaded fake personnel files into the NATO system, and manage to identify one of the fakes: Czech officer Martin Novotny. Section 20 tracks Novotny to an overnight train to Berlin and heads to the site via helicopter. When Novotny realizes he's being pursued, he unleashes an aerosol cartridge of smallpox into a carriage of passengers. The boys are able to stop the speeding train before it reaches a populated city. The train crashes, killing Novotny, and the infected passengers are quarantined.
After all the action, the boys visit Ester. When they arrive at the safehouse, they find her minders killed and the door left ajar. Scott and Stonebridge discover Qassem inside with an unlikely partner -- Leo Kamali.