In Young’s double-slit experiment, as traditionally understood, the result appears on the screen and occurs post-slit—the third and final part of what we call the trilogy of Young’s experiment with respect to the slits. In modern quantum theory, where a photon or electron is said to interfere with itself, the splitting prior to the slits is the only known first part of this trilogy. However, the second part—the in-slit dynamics—remains empty, overlooked, or treated as trivial. In this paper, we follow up on our previous concept of the interference trilogy—a local, point-based account of what interferes, when, and how—to establish the need for a global trilogy, especially the missing in-slit narrative. We show that any splitting before the slits and reunion afterward must satisfy strict timing conditions. These constraints imply that the in-slit region cannot be ignored. Finally, we present our nontrivial in-slit dynamics, supported by laser experiments using an asymmetric single slit. This is Part II of the trilogy. Extends the logical structure and implications, keeping the focus on existence rather than modeling detail; connects the conceptual picture to simple, reproducible observations. Related: DIY optics preprint (DOI 10.5281/zenodo.17059754) and report (DOI 10.5281/zenodo.17118246 )