Through the characters of Hal, Mercedes, and Charles, London seeks to highlight the arrogant, disrespectful attitudes of many town and city dwellers towards nature and the damaging consequences that can follow.
It's clear from the start that this hapless trio is out of its depth in the frozen wilderness of the Klondike. These people make basic mistakes that betray their ignorance and lack of experience, such as taking too much stuff with them and miscalculating how much food they will need.
The biggest mistake they make turns out to be their last. Despite being warned by John Thornton not to, Hal, Mercedes, and Charles attempt to trek across a frozen river. Almost inevitably, the ice cracks beneath them, and these three foolish individuals sink to their deaths in the icy water.
The fate of these people is intended by London to serve as a salutary warning to those people who arrogantly think they can conquer nature, seeing it as some kind of challenge to do so. This is the same warning that London gives to the reader in his short story “To Build a Fire,” in which another arrogant, unprepared individual comes to grief in the icy wastes because he didn't respect nature.