We present details of a new plasma diagnostic system based on terahertz (THz) time-domain spectroscopy that simultaneously provides high time resolution and THz bandwidth coverage. The latter attribute allows studying wide ranges of plasma density variations without incurring opacity. The system utilizes imaging of the transverse fluorescence to estimate the plasma density profile, and then applies an rms error minimization technique to establish peak values of the electron density and collision rates as a function of delay time throughout the plasma evolution.