Suddenly

Frank Sinatra is hired by somebody to assassinate the President, and opts to do it by sniping him from a house on a hill that overlooks the train station where the President will be disembarking. The house isn't empty of course, so he and his cronies end up with first three, then four, and eventually five hostages. The hostages range from an old man who used to be in the Secret Service, to his widowed daughter-in-law and her young son, the town sheriff (Sterling Hayden, who apparently only took acting jobs when his boat needed fixing), and eventually the TV repairman.

Sinatra's playing a World War 2 vet who's a little off his nut. He found a certain knack for killing in the military, and was even praised and rewarded for it (he keeps mentioning his Silver Star), so he keeps at it. It made him somebody worth noticing, after all. The assassination has no political meaning to him beyond that. In fact, he thinks the people paying him are suckers because you kill the President, so what? They just make the Vice-President the President. Big deal.

There's time to waste, so the sheriff spends a lot of time trying to argue with Sinatra, to distract him and gain an opening. he also spends a lot of time arguing with the mom, who he's sweet on, but she's extremely protective of her son, and bitter that her husband died fighting overseas. She's not big on the idea of there being something noble about giving your life in service of a cause, so getting her on board to make some suicidal push to stop Sinatra is a challenge. But they manage it, because it's the duty of every good American to die when ordered to.

There's some weird, abrupt cuts, like they did the most slapdash, rushed edit possible between one character speaking and another. They have Sinatra speak directly to the camera when he's monologuing sometimes, which is just weird. So the production values are kind of junk, but there's a fair amount of tension.