The dynamic behavior of the failed steam generator tube plug of an operational plant in 1989 can be understood through an analysis that solves the energy equation of the broken plug top and computes its kinetic energy as a function of elevation in the tube. The computed high kinetic energy of the plug top when it reaches the U-bend of the tube can exceed the work required to penetrate the tube wall at that location. If the inlet flow area at the bottom of the tube is small, the plug top exhibits an interesting stop-and-reacceleration behavior.