With this case study of a French Catholic missionary in Tonkin, as north Vietnam was called during colonial times, I wish to illustrate how individual agency might find ways of expression in spite of severe hierarchical clashes. Put differently, I wish to critically reflect on a case where the scientific inclination of a given missionary in the field became his calling. The surprising result being that instead of meeting the expectations of conformity or “toeing the line” of Catholic conversion duties, he intentionally took a significant step outside dogmatic morality.