Fringe: Season 2, Episode 13

2 - 13: What Lies Below

After a man's veins erupt with blood infected with a lethal virus, investigating agents Olivia and Peter are quarantined with the rest of those exposed. Walter, Broyles and Astrid race to find a cure. After discovering during a test that Olivia and Peter are not infected, it later becomes clear that Peter is when his nose starts to bleed before he reaches the outdoors. While the virus overtakes Peter's health and sanity, Walter becomes more and more distressed as he fears losing his son again, and accidentally blurts out that he "can't let Peter die again" to Astrid. The two of them eventually find a panacea derived from horseradish, and with the help of Astrid, they cure the disease in Peter and anyone else who was infected. Astrid later approaches Walter and asks what he meant when he said he couldn't let Peter die again, to which he responds by saying "some things are better off left alone."