Young J. Quincy Pennant, Edmond O'Brien, is a brilliant but absent-minded scientist who has developed an explosive method that can direct a shock wave past obstructions to strike a distant target. He is sent to a rapidly growing “powder town” built around an arsenal and munitions factory, a place where population growth attracts criminals, foreign spies, and saboteurs.
Pennant is housed in a boarding house where he shares a room with five female entertainers who work at a local casino run by gangsters. The rambunctious and physically imposing Jeems O'Shea, head of the powder monkeys at the factory, Victor McLaglen arrives with his sycophant Billy, and his rough manner quickly disrupts the house as he chases the women around and playfully molests them. Pennant, unfazed, casually punches O'Shea, sending him tumbling down the stairs to an astonished audience.
When Pennant reports to the factory, he formally meets O'Shea again, who is surprised that Pennant isn’t the literal giant he imagined. He tries to intimidate the scatterbrained scientist, who is too absent-minded to understand the threats, and O'Shea interprets this as a sign of quiet courage—an attribute highly valued among munitions workers. As Pennant hones his shock-wave concept, he is given his own pistol and paired with O'Shea as a bodyguard.
Pennant falls in love with Sally Dean, Dorothy Lovett, who also lives at the boarding house. Unknown to him, she has been hired by the gangster boss Oliver Lindsay, Damian O'Flynn, to steal the explosive formula. The romance and the double dealings weave a tense tension through the story as the stakes keep rising.
Things come to a head when O'Shea takes the naive Pennant out for a night on the town. The muddleheaded scientist remains oblivious to several assassination and abduction attempts by enemy agents, and O'Shea reads this as a display of coolness. Pennant is introduced to his first alcoholic drink, and after a lucky break at the casino, he breaks the bank, winning $900 and earning the admiration of all the women. The gangsters respond with a brawl to reclaim their money, but O'Shea steps in and demolishes both the attackers and the casino itself.
Back at the factory, Dr Wayne, who runs the munitions plant, threatens to fire Pennant when he learns of the gambling and his liaison with the casino women. Yet Pennant insists he wants to keep seeing Sally, to whom he has entrusted the secret formula for safekeeping. Meanwhile, the enemy agents and Lindsay press on, hoping to seize the formula.
The gangsters return to the factory to obtain Pennant’s formula, only to be blocked by O'Shea. A desperate assault ends with a timer set on a cart loaded with explosives in the dynamite room, and Pennant and O'Shea are bound as the danger looms. They manage to escape in time to push the explosive cart down the hill, where it crashes into and destroys the getaway car, thwarting the saboteurs.
Having survived the ordeal, the two heroes reunite with Dr Wayne and the girls, who embrace them in a hopeful kiss. The movie closes with Dr Wayne revealing the coded formula written on a wall in the factory office, a quiet reminder of the peril—and the ingenuity—that kept them alive.
