Fritz is outside his cave home at Tentholm with his parents when he sees something very strange. He describes it as something that draws up into coils and then raises itself into the air before it sinks back down and coils its body again. As Elizabeth hears his description, she becomes worried and calls out to the other boys who are playing farther away from the house. The family members then retreat into the cave, retrieve their guns, and aim them out the upper windows. The father takes out his spyglass to get a better look at the creature. It is, as he has feared, a giant snake.

The boys want to shoot the snake immediately. However, the father cannot allow this; so far the reptile has done no harm. Besides, the snake is too far away, and all the members of the family are safely in the cave where the snake cannot get them.

As the father continues to watch, the snake crosses the bridge. When it comes closer, the father hastens to assist his family in securing all the lower-level doors and windows. The snake, though it moves erratically, continues to slither forward, coiling and uncoiling and frequently raising its head to look about. Suddenly, the family can no longer contain themselves, and everyone shoots at the snake. None of them hit their mark. The snake then slithers quickly into the marsh and disappears.

At first the family is glad that the snake has left. However, knowing it is near the house is discomforting. The father insists that no one leave the house. For three days, the family remains alert for any signs of the snake. The huge reptile does not return. The father determines by the snake's size that it is is a boa. If it were not for the restlessness of the ducks and geese who refuse to enter the marsh where they normally go each evening, the father would have thought that the snake had left their territory.

As the days go by, the tension does not ease. The father is frustrated by his inability to free the family of this concern. However, there is no way of finding the snake in the marsh without putting himself in the greatest of dangers. The snake, at this point, has all the advantages.

After so many days have gone by that the animals are in great need of fodder, the father is forced to let his animals out so that they might be taken to one of the other residences for food. As he and his sons are tying the animals together so that they can travel in a group, their donkey, Grizzle, becomes so overwhelmed with his sudden freedom that he breaks loose and runs into the marsh. Shortly afterward, they hear a loud bray and rush to see the great boa wrapped around the donkey's body.

The father realizes that nothing can save the donkey. It is too late. He and his sons watch how the snake continues to crush the donkey until Grizzle's body is mere pulp. Then the boa swallows the donkey whole. Only afterward, when the snake is so full that it can barely move, does the father shoot the serpent and kill it. When they are sure the snake is dead, the boys suggest that they stuff it and place it in their museum.