  Using 281 pb^{-1} of data recorded by the CLEO-c detector in e^+e^-
collisions at the psi(3770), corresponding to 0.78 million D^+D^- pairs, we
investigate the substructure of the decay D^+ to pi^- pi^+ pi^+ using the
Dalitz plot technique. We find that our data are consistent with the following
intermediate states: rho(770)pi^+, f_2(1270)pi^+, f_0(1370)pi^+, f_0(1500)pi^+,
f_0(980) pi^+, and sigma pi^+. We confirm large S wave contributions at low pi
pi mass. We set upper limits on contributions of other possible intermediate
states. We consider three models of the pi pi S wave and find that all of them
adequately describe our data.
