In Hong Kong, Circa 1940s, World War II, private detective Yoyo and friend Inspector Teddy Robin join a resistance group during the Japanese occupation. They aim to steal back the formula to the atomic bomb from a war traitor and transfer it to an American before the formula ends up in the Japanese emperor's hands.

Little Tiger ventures from the sticks to the big city in search of his cop brother Big Tiger, an honest cop working in a corrupt system. Surmising that life in the police force was not his cup of tea, Little Tiger joins the Swallow Acrobatic Troop, which he excels in because of his kung-fu prowess. When a band of thugs from Chin Hung-yun's group attacks the troop, Little Tiger not only handily fights them back but also infiltrates their organization to destroy them from the inside. Meanwhile, Big Tiger's old flame Mary returns from America to join the revolutionaries. Big Tiger soon finds himself torn between his love of this girl and his orders to arrest all revolutionaries.