Numerous computational and spectroscopic studies have demonstrated the decisive role played by nonadiabatic coupling in photochemical reactions. Nonadiabatic coupling drives photochemistry when potential energy surfaces are nearly degenerate at avoided crossings or truly degenerate at unavoided crossings. The dynamics induced by nonadiabatic coupling are challenging to comprehend; here we describe a versatile one-dimensional model that is numerically tractable and illustrates fundamental aspects of nonadiabatic quantum dynamics. This model reinforces and builds on concepts taught in graduate-level quantum mechanics and therefore should be accessible to advanced undergraduate and graduate students. We use the model to demonstrate how the local topography of an unavoided crossing can affect a photochemical quantum yield.