[alien attack] trying to identify a serial

Can anyone help me out here? I’m trying to identify a serial from the 50s, it would seem, made for television, possibly in six or so parts and in B&W. I was a child when I saw it years after it was made and was annoyed when my parents at first wouldn’t let me watch the exciting finale but then I prevailed on them to let me.

I’m very, very hazy about it. The hero was, I think, like all the short-haired heroes of the time, in a suit, an ordinary man who’d ‘discovered the truth’. The tone was dark and menacing and it was unlike any other I’d seen, with the exception of the classic ‘The Day the Earth Stood Still’. There wasn’t the least amount of corniness about it.

It turned on the fact that if something hadn’t been agreed to by a certain time, the flying saucers would obliterate the earth. Somehow it came down to one particular saucer planning to hover over maybe the Empire State building at maybe 12 noon and if that couldn’t be prevented, then zap!

Once the final pieces fell into place and the hero realized what was really going down, he had to somehow race to the city to warn someone. Like all of those more innocent days, I naturally assumed he’d clinch victory at the last second. He didn’t. He never made it on time and the saucer came to the building, hovered and zap! The screen went blank. I can remember being shocked to the core because in every other film and TV show I’d ever seen, good had always triumphed.

Any thoughts?