I actually think that much of the answer to this question lies in whether one believes in the power of human redemption or if there is a predisposition to deny that these transformations are possible. The fact that the story or plot is historically valid indicates that believability is more of an issue of personal temperament and personal disposition more than anything else. If an individual does not believe in Schindler's transformation and evolution into a virtuous man, then there is a greater likelihood that the change is not necessarily believable. The larger issue might be how one views human nature. The Holocaust was a time when most theories that ascribed negative qualities to human consciousness were sadly proven true. The theories that gave human beings some level of credit for being honorable creatures or for representing healthy notions of solidarity or community were strained. Yet, there were examples of individuals who upheld such beliefs. Schindler would be seen as one of them. In the end, the plausibility of the script has to comes down to how an individual views such a paradigm.