Self-assembly has proven a powerful means of preparing structurally intricate nanomaterials, but the mechanism is often masked by the common one-pot mixing procedure. We employed a flow system to study the steps underlying assembly of a previously characterized molybdenum oxide wheel 3.6 nanometers in diameter. We observed crystallization of an intermediate structure in which a central {Mo(36)} cluster appears to template the assembly of the surrounding {Mo(150)} wheel. The transient nature of the template is demonstrated by its ejection after the wheel is reduced to its final electronic state. The template's role in the self-assembly mechanism is further confirmed by the deliberate addition of the template to the reaction mixture, which greatly accelerates the assembly time of the {Mo(150)} wheel and increases the yield.