- The Control Voice: Can the fate of mankind hinge on a single act or are we helpless to change our destiny?
A man attempts to stop a plague by traveling back in time and killing the plague's patient zero.
- The Control Voice: The answers to mankind's future problems can be found within, so long as there are those among us willing to make the sacrifice.
- Eiffel Tower Effect: This episode, which is set in New York City, opens with a shot of the World Trade Center.
- Meaningful Name: The Gehenna Strain wiped out more than three billion people worldwide. Gehenna is a valley in Jerusalem which is described as being cursed in the Book of Jeremiah as it was used by the kings of Judah to sacrifice their children by fire.
- Patient Zero: The Gehenna Strain, a virus which has killed more than three billion people by 2015, began when three DNA strands came into contact with each other: the first strand belonged to Dr. Josh Mandel, the second to a young boy and the third to Amy Barrett, who became Patient Zero. Colonel Beckett is sent back in time to 2001 to kill Amy, thereby preventing the outbreak of the Gehenna Strain and saving almost half of Earth's population, including his wife Sarah and daughter Catherine. However, it turns out that Beckett's presence in 2001 has altered history. He himself has become the new Patient Zero as he came into contact with the three DNA strands. Beckett allows his friend and fellow time traveler Colonel Faber to kill him so that the outbreak will never happen.
- The Plague: The Gehenna Strain killed 500 million people within two months of its outbreak in New York City in 2001. By 2015, the death toll stood at more than three billion. Clips from "The Vaccine" are used to represent the devastation.
- Set Right What Once Went Wrong: A time-traveling assassin killing certain people with a fast-acting poison before the strains of viruses they're carrying can combine in Patient Zero and start a pandemic that will kill most of humanity. Each time he goes back and is told that nothing has changed. He eventually realizes that he has to kill Patient Zero, who turns out to be a pretty woman, and he hesitates, resolving to prevent her from contacting the people with the strains. At the end, a colleague of his goes back in time and explains that the assassin is the one who is now Patient Zero, as his attempts to keep her away from the infected resulted in him creating the plague within himself. He voluntarily lets himself be poisoned in order to keep his future family safe.
- Time-Travel Episode
- Tuckerization: Quisling is named after the infamous Nazi collaborator Vidkun Quisling. This doesn't actually prove to be a Meaningful Name.
- 20 Minutes into the Future: This episode involves a soldier named Colonel Beckett from 2015 who travels back in time to 2001 to stop the outbreak of a plague which killed billions of people, including his family.
- Whole Plot Reference: This episode is one to 12 Monkeys.