We study the effects of plasma sheet variations on the fast initial ring current (RC) decay during the March 31, 2001 storm. Previous studies have focused on the effects of enhanced plasma sheet density on RC buildup during the main phase of the storms. Using model simulations and energetic particle data from the instruments on CLUSTER and LANL, we demonstrate that the drop of plasma sheet density during the early recovery phase is important for the fast RC decay. In fact, an additional ~50 nT decrease in Dst index occurs if this drop is not taken into account. We also demonstrate that there is no preconditioning of the RC during this storm period. The second enhancement in the convection electric field is not more effective in building the RC than the first in the case of the same magnetospheric plasma inflow on the nightside. This study shows that the fast initial RC decay is controlled not only by the decreased convection electric field, dayside outflow through the magnetopause, and internal loss processes, but also by the time-varying nightside inflow of plasma from the magnetotail. All factors should be considered for accurate predictions of RC decay timescales and Dst index.