NorBAC has been taken over by a joint command to deal with a catastrophic smallpox scenario. Bob tries to find a cure for his dog that is suffering from histiocytosis. Jill comes clean with David about taking medication for her panic attacks. David discovers that the earlier "Miranda Virus" outbreak might be linked to the U.S. government's secret bio-chemical lab and that whoever manufactured the "Miranda Virus" may possess another, even more lethal, biological weapon.

David and Jill make a trip to Colorado where a rampant SARS-like outbreak is occurring. David is in shock when he discovers that the outbreak is Spanish Flu, the virus he dug up in Nunavut. Racked with guilt, he walks into the path of a car and is sent to hospital in a coma before he tells anybody his fears. Mayko Tran heads to Colorado to try to find Patient Zero.

Jill goes to Canada to see if the Spanish Flu body is still in the ground. David recovers from his coma and limps back to the office. Daisy recognises the person who created the Miranda virus and infected her baby. David identifies the vector for the Spanish Flu epidemic and tracks it with GPS.