We describe and analyze in detail the coronal mass ejection of 18 August 1980, using images from the coronagraph on the Solar Maximum Mission (SMM) satellite. The event occurred at the site of a large coronal helmet streamer and evolved into the three-part structure of a bright frontal shell, followed by a relatively dark space surrounding a bright filamentary core as seen in many mass ejections of the SMM epoch. The bright core can be identified as material from a prominence whose eruption was observed from the ground; this identification is based on (1) the looplike and filamented appearance of the core, (2) its motion along a trajectory that is a good extrapolation of the motion deduced from ground-based observations of the prominence eruption, and (3) direct observations of H&agr; emission when the core is in the coronagraph field of view. The mass of the frontal shell is equal to that of the coronal helmet streamer (to the ~30% accuracy with which mass estimates can be made), indicating that the shell is the coronal material previously in the helmet streamer, displaced and set into motion by the erupting prominence and surrounding cavity. The mass ejected in the bright core (or prominence) is estimated to be ~50% larger than the ''coronal'' material in the frontal loop. The total mass of 2.5¿1015 g and energy of 5¿1031 ergs estimated for this mass ejection are both greater than in typical ejections of the Skylab era but are comparable to the average mass and energy in an interplanetary shock wave.