Teddy O’Neill, an Ulster lad, is in love with Eileen. Phadrig O’Toole, a rogue, is the leader of “The White Boys” gang. He and Red Harney plot the ruin of Teddy O’Neill. Phadrig poses as a seer and Barney, being poor, goes to him to learn a way to make his fortune. Phadrig at once sees a chance to work his downfall. He has Teddy draw a card from the pack, which happens to be the three of diamonds, and then mysteriously tells him that if Teddy will meet him in the glen at night fall he will have some news for him. The credulous Teddy places the card in his pocket and turns his steps homeward. Later Eileen sees the card sticking from his pocket and, thinking it is there for no good purpose, slyly removes it unknown to Teddy. Meanwhile, Phadrig and Red Barney have met a passing British army patrol who are out on the trail of the “White Boys.” They inform the soldiers that Teddy O’Neill is the leader of the “White Boys” and carries upon him the three of diamonds as the sign. The officers force Barney to accompany them to O’Neill’s cottage, where Teddy and Eileen are together. Teddy is searched by the officers, but no card is found. Seeing the plot, Eileen slyly slips the card into Red’s pocket, where it is found by the officers. Barney is placed under arrest. Phadrig learns of the downfall of his companion in crime, and is exceedingly angry. He has one of his men plant treasonable papers in the thatch of the O’Neill cottage; informs the officers, who find the papers and place Teddy O’Neill under arrest and bear him off to prison. Teddy is found guilty by the court and sentenced to be shot. A British captain has, meanwhile, been following Phadrig, who has come under suspicion. The captain’s life is saved by Eileen. Later Eileen succeeds in helping Teddy to escape from the prison, but he is again captured and ordered executed within five days. Eileen and the captain are captured by the revengeful “White Boys” and are imprisoned in two vaults of a ruined abbey. Eileen succeeds in escaping and the soldiers return with her. A stiff fight with the “White Boys” follows, in which Phadrig is fatally wounded and before dying confesses. The party then set out in haste for the prison to save Teddy, who is even then being prepared for execution. They arrive in the nick of time and Teddy O’Neill is free to return to his Irish sweetheart.