Magnetic reconnection is a multi-faceted process of energy conversion in astrophysical, space and laboratory plasmas that operates at microscopic scales but has macroscopic drivers and consequences. Solar flares present a key laboratory for its study, leaving imprints of the microscopic physics in radiation spectra and allowing the macroscopic evolution to be imaged, yet a full observational characterization remains elusive. Here we combine high resolution imaging and spectral observations of a solar flare at multiple wavelengths with data-driven magnetohydrodynamic modeling to study the dynamics of the involved plasma from the current sheet to the plasmoid scale. The flare resulted from the interaction of a twisted filament and chromospheric fibrils. By inferring the reconnection to be fast and mediated by plasmoids, the relevance of this reconnection mode is found to extend beyond hot flare plasmas to such cool structures in the chromosphere, which have many analogs in astrophysical objects.