Story of Mankind was indeed a campy mess, redeemed and made sort of watchable by Vincent Price [Mr. Scratch, the prosecutor], Ronald Coleman [the Spirit of Man, counsel for the defense], and Cedric Hardwicke [the Judge]. Man has discovered the "super H-bomb" sixty years before we were meant to, and don't realize that it will in fact cause our extinction. The trial is called to decide if humanity is worth intervening on behalf of or should be left to destroy itself. Price and Coleman tour history plucking examples to support their cases, which is where the cameos come in... most of them were pretty embarrassing, but it was fun in a loopy way. The end is very much like the original
Day The Earth Stood Still...