As the drama that began on "Third Watch" continues, the rest of the team joins Conner and Natalie in New York as they race against the clock to determine what the deadly virus is that has landed paramedic Carlos Nieto and the robbery suspect that he was attending to in the ICU. Conner teams up with Detective Faith Yokas to search for the robber's accomplice, whom they hope will lead them to the source of the disease. Meanwhile, Eva tries to console Holly as she waits to learn if Carlos is going to live or die. As more infected people arrive at the hospital, the team must race against time to track the source of the mysterious illness before more patients die.

In the first part of the season finale, Dr. Connor and his team confront their greatest challenge when they enter a violent Central American country to combat a mysterious disease. Their efforts are further endangered by a massive new earthquake -- and a murderous jungle insurrection that has Connor, Miles and Frank held hostage. Elsewhere, Dr. Durant struggles to salvage lives in a wrecked clinic and isolate the deadly contagion while Eva tries to help an isolated and terrified boy.