There were once two men who wanted to kidnap someone for ransom. They chose a boy called Johnny Dorset. His father, Ebeneezer Dorset, was a rich man, able to pay the ransom.
They camped out in a cave and they played games. The boy took it seriously and hurt the two men. for example in this picture, Johnny pushed Bill into the fire.
The two men decided to write a letter to the boy's father to let him know he has to pay the ransom to get his son back. Sam, the more business-like out of the two men posts to letter at Poplar Cove. Bill, the other man, stays with the boy.
Bill and Sam got a letter back from Johnny's father. It says to bring Johnny at night, because the neighbors think he is lost.
The two men return Johnny to his father, but Bill and Sam pay the boy's father, instead of the father paying them. Johnny bawls because he enjoyed camping out in the cave and doesn't want to go home. In real life, it's like he kidnapped Sam and Bill, not the other way around.
The theme is 'be careful what you do, for there will be consequences.' In the text it states, "' ... and be legging it trippingly for the Canadian Border.'" Bill and Sam tried to kidnap Johnny, and that's why Bill is saying that he will be crossing the Canadian Border- to get away from trouble.