Episodic memory relies on coordinated neural dynamics across time and space, supported by a spatiotemporal scaffold in the hippocampal–entorhinal network. This scaffold aims to sustain episodic memories and originates from grid cell modules with toroidal dynamical states, which are connected to hippocampal cells and sensory-processing inputs. Low-dimensional velocity signals shift grid phases, while grid–place cell interactions encode and retrieve spatial–episodic information. Beyond spatial coding, a minute-scale (<0.001 Hz) ultraslow oscillation in the medial entorhinal cortex (MEC) has been identified during 1D walk with mice running on a rotating wheel, though its behavioral-scale impact remains unclear. We developed an entorhinal–hippocampal model that incorporates this oscillation during spatial exploration in a 2D arena. Using computational simulations through detailed numerical investigations, results showed that the ultraslow oscillations synchronize entorhinal inputs, stabilize hippocampal spatial scaffolds, and enhance associative binding. Without them, input timing desynchronizes, spatial scaffolds destabilize, and the formation of episodic memory degrades. We discuss that when adding slow-oscillations, spatial–temporal recall accuracy improves. This work bridges theoretical and experimental findings, revealing how slow MEC rhythms integrate temporal and spatial organization to support episodic and associative memory.