Actually, we have two. Andrew Winter and another reader of this site named Clark almost simultaneously wrote to say that the serial I was remembering was The Lost Planet and I was recalling Chapter Seven. In an upcoming Childhood Memory here, I'll tell you why I asked about this.
The Lost Planet, I'm told by Wikipedia, was a 1953 serial produced by Columbia Pictures. Don't bother seeking it out. It was pretty bad when I saw Chapter Seven in 1960 as the warm-up to the Jerry Lewis film, Cinderfella, and it hasn't apparently improved with age. I'll get to the story of that afternoon at the movies in a week or two here. Thanks, Andrew and Clark.