In the wake of two recent developments in stem cell research, it is a fitting time to reassess the claim that stem cells will radically transform the concept and function of medicine. The first is the U.S. Food and Drug Administration's decision in January 2009 to approve Geron Corporation's Phase I clinical trial using human embryonic stem cells for patients with spinal cord injuries. The second is the National Institutes of Health's decision to permit federal funding of research using donated IVF human embryos in their July 2009 Guidelines on Human Stem Cell Research. We are now poised to see whether stem cell research can deliver on what it promises. However, what exactly does it promise and how? Moreover, who is doing the promising? Turning to the use of metaphor can help us to answer these questions and enable us to develop a better appreciation of the unique features of promised stem cell therapies. Indeed, metaphors have exerted profound influence in medicine, and it is fitting that we seek new metaphors for new therapies where appropriate. In this case, other metaphors such as magic bullets or the Holy Grail cannot capture what is unique about stem cells. Accordingly, I propose a new metaphor: the stem cell superhero. Stem cell superheroes are characterized by the following traits: they are seemingly capable of fighting the evil of virtually all disease (unlike "magic bullets") and they seem to be our only hope of doing so, although to summon them we must make difficult moral choices. In the course of assessing the merits of three recent yet covert references to the superhero metaphor, I conclude that this powerful new paradigm employs a problematic logic (i.e., we cannot know that something is "our only hope"), but that the aspiration as such is a good one.