In 1948, after the Northeast was liberated, enemy sabotage began. The Public Security Bureau learned that spy Li Tianmin had infiltrated the city. Bureau chief and Captain Zhang quickly deployed personnel to capture him. Li Tianmin evaded key questions during interrogation. Clues were found on a milk bottle cap delivered by a boy, who passed it to another spy, Zhou Shaomei. Under surveillance, Zhou Shaomei, fearing Li Tianmin’s capture, killed the boy to erase evidence. Intelligence personnel mobilised the masses, pressuring the hidden enemies. Many deceived individuals provided clues, ensnaring the enemies in the people’s war.