The three boys, whose names we already know, have just finished their junior year in high school, and while wondering what to do for their last vacation together, are told by Garry’s father, a wealthy timberland owner, that he will get them places in the Maine Forest Ranger Service.
The duty of the men on this service is to patrol the forests constantly on the lookout for forest fires, and if any are discovered, to report them promptly, as well as trying to do what they can to put out the blaze.
A short way up the river near their home the boys have a shack, and one afternoon they go there to find it occupied by three tramps who seize Phil and Garry. Dick makes his escape and brings help from town.
It is discovered that the three are badly wanted for robbing postoffices, and the chums receive a sizable reward for their capture.
This money they use, or rather a part of it, to outfit themselves for the forest. On the way to their work they make friends with one Nate Webster, an old Maine guide who later does them many a good turn. They also incur the enmity of a French Canadian halfbreed named Jean LeBlanc, who finds out that one is the son of Mr. Boone, a man he has hated for years.
While on their patrol they meet a party of New York campers, and are instrumental in saving the little daughter of Mr. Graham, head of the party. A day or two later LeBlanc captures the child and intends to hold it for ransom.
A queer, unknown individual writes them constant notes informing them of what is going on. This man they call the Hermit. Garry finally rescues little Patty, and LeBlanc is captured only to break out of jail later on.
The halfbreed continually crosses their trail, causing them all kinds of trouble, and several times endangering their lives. He sets fire to the forest, and almost causes the death of the boys and a party of motion picture people who are taking a forest picture. Having checkmated him, they set out to find the Hermit, following a clue given them in a cipher note, and Dick succeeds in finding the old man, who is evidently a gentleman, yet seems to be a trifle unbalanced mentally.
So well do they succeed at their work in the forest, that when a gang of fur smugglers start operations on the Canadian border, the Chief Ranger recommends to the Customs authorities that they be sent to help.
They are instrumental in aiding an old man who has been caught in a disused bear trap in the woods, and meet his granddaughter Ruth. They get evidence against the smugglers after some thrilling escapes, and find that part of the contraband of the smugglers includes some wonderful jewels. LeBlanc is mixed up in this, but makes his escape across the border, where the American Customs authorities cannot pursue him.