One-dimensional arrays of submicrometer rectangular holes in 200 nm thin gold films are investigated using electron energy loss spectroscopy combined with scanning transmission electron microscopy (STEM-EELS). Improved energy resolution, down to 0.11 eV, is accomplished in our monochromated transmission electron microscope, allowing the reliable quantification of signals at loss energies as small as 0.43 eV. The standing-wave resonances of individual holes are thus investigated at nanometer-scale spatial resolution, focusing in particular on the effect of neighboring holes. We show how the coupling between holes is facilitated by surface plasmon polaritons (SPPs) propagating on the top and bottom surfaces of the separating metal-film strips. Thus, complex spatiotemporal coupling dynamics emerges, characterized by strong interslit interactions and a phase that can be controlled by varying the width of the metal strip between adjacent holes. Applying real-space real-time numerical simulations and exploiting the short interaction time of 300 keV electrons with the thin gold film, we reveal intriguing features of these ultrafast coupling mechanisms, including unusual line-narrowing and marked SPP signal enhancements in the corresponding EEL spectra. Complementary aspects of the far-field and near-field components in the SPP eigenstates are further demonstrated. Our combined analysis effectively equips STEM-EELS with an excellent temporal resolution and further yields a consistent description of the entangled femtosecond-scale SPP dynamics.