Bert and Harry were so surprised at Frank's sudden call, that, for a few seconds, they did not know what to do or say. George Smith, the boy in the store, was also startled. He stood with the bad twenty dollar bill in his hand, wondering where the warning voice had come from. And then Frank showed how quick he could be. "Hurry up!" he whispered to Bert and Harry. "One of you slip around and lock the front door, and the other one lock the back. Then we'll have this man trapped, and maybe I can make him pay back the money he got from me. Quick!" "I'll go to the front door!" exclaimed Harry. "And I'll lock the back one!" said Bert. The man, who had heard Frank's call from behind the pile of boxes, must have known something had gone wrong with his plan to cheat. "Never mind about the butter," he said quickly. "I guess I won't buy any after all. Just give me back my twenty dollar bill, and I'll get along." "Oh, no, you won't!" exclaimed Harry, as he slipped around some barrels. Quickly running to the front door, the country boy locked it, and stood in front of it. "Hurry! Give me my money back, I tell you!" cried the man to George, who stood near the cash drawer, not knowing what to do. "Don't you give it to him!" advised Frank, stepping out. "Lock the back door, Bert," he called. "I have!" cried the older Bobbsey boy. The man started to run behind the counter, to find a way out, but he was too late. Bert had locked the door, and taken out the key. "Let me out of here!" cried the stranger. "Let me out!" Bert and Harry were somewhat frightened, but Frank was brave. "You don't get out of here until you pay back the twenty dollars you cheated out of Mr. Mason," he said. "I don't know anything about any Mr. Mason!" the stranger said. "I want my twenty dollar bill back, I won't need any butter to-day!" "Don't give him that money!" cried Frank to George. "It's bad, and if you give it to him, he'll try to cheat someone else with it." "I'll fix you!" cried the man. But at that instant there was a rattling sound at the front door, and Harry, looking through the glass panels, saw Mr. Mack, the store owner, and two or three other men outside. "What's the matter? What has happened? Why am I locked out of my own store?" cried Mr. Mack, rattling the knob. "There's a cheat in here!" cried Harry, unlocking the door. "There he is!" he went on, as Mr. Mack rushed in. "That man tried to pass a bad twenty dollar bill on your boy," went on Harry. "He did, eh?" cried Mr. Mack. "Well, I'll see about that!" "You let me go!" exclaimed the strange man. "I haven't done anything. I wanted some butter, but I changed my mind. There isn't anything wrong in that. Give me my twenty dollar bill and I'll go!" "Oh, no, you'll not--not until you explain," said Mr. Mack, and he caught the man by the arm. Then the man tried to break away.