In the gamma-echo technique a radioactive source is moved, with respect to a nuclear-resonant absorber, during the lifetime of first-excited nuclear state. This introduces a phase shift between the source radiation and the radiation from the absorber. If the source is moved abruptly, introducing a pi phase shift, the time-dependent intensity shows a sharp increase in the intensity at that time, the "gamma echo." Using the recently developed one-dimensional quantum-mechanical model, based on the technique developed by Heitler and Harris, the gamma-echo effect is seen to be a phase-shift-induced transparency. A closed-form solution for the time-dependent transmitted intensity has been obtained. The solution has the form of a sum over coherent paths that the radiation takes in going from the radioactive source through the absorber to the detector. The model shows that the sharp increase in the intensity, the "gamma echo," at the time when the source is moved abruptly is due to constructive interference, starting at that time, between the source radiation and the radiation from the absorber. The exact. form of the gamma-echo spectrum depends on the movement of the source. Shapes having multiple peaks are possible. All shapes can be found using the one-dimensional model.