This episode picks up the story at another moment of buoyant optimism in the cancer world: Scientists believe they have cracked the essential mystery of the malignant cell and the first targeted therapies have been developed, with the promise of many more to follow. But very quickly cancer reveals new layers of complexity and a formidable array of unforeseen defenses. In the disappointment that follows, many call for a new focus on prevention and early detection as the most promising fronts in the war on cancer. But other scientists are undeterred, and by the second decade of the 2000s their work pays off. The bewildering complexity of the cancer cell, so recently considered unassailable, yields to a more ordered picture, revealing new vulnerabilities and avenues of attack. Perhaps most exciting of all is the prospect of harnessing the human immune system to defeat cancer. This episode includes patients Doug Rogers, a 60-year-old NASCAR mechanic with melanoma, and Emily Whitehead, a six-year-old child afflicted with leukemia. Each is a pioneer in new immunotherapy treatments, which the documentary follows as their stories unfold. Both see their advanced cancers recede and are able to resume normal lives.