S1·E17 Flash of Darkness

This "what-if" episode imagines what would happen in the event of a nuclear strike on Los Angeles. Dr. Styner and his colleagues are at a warehouse outside the city for a training session, but must try to make their way back to the devastated city after the nuclear attack. Along the way they try to treat and save as many injured and radiation-poisoned patients as they can with what little medicine they have and are forced by circumstances to decide which patients can be saved--and which ones they'll have to let die.