Bobbie learns that it is impossible to predict the consequences of one small act of carelessness.
In the poem, Bobbie goes mountain-climbing with his friend, David. The two friends climb a mountain in the Ramparts Range, Sundance Peak, Mount Inglismaldie, the Fortress, and the Finger in the Sawback Range. They also climb Mount Gleam, but it is a fictitious mountain. It is while climbing the Finger that David loses his balance and falls fifty feet. The accident occurs when David reaches out to steady Bobbie; in trying to save his friend from certain death, David loses his own foothold.
Later, we learn that the accident happened because Bobbie neglected to test his foothold as he climbed the Finger. Thus, he blames himself bitterly for David's suffering. For his part, David refuses to let Bobbie torture himself. He tries to get Bobbie to see that an unintentional mistake does not make someone a villain.
David bleeds profusely from the fall; from the poem, we can deduce that David has been paralyzed by his accident. Both Bobbie and David know that David will not leave the mountains alive; in the end, David asks his friend to do the unthinkable. He begs Bobbie to push him over the ledge so that he can end his suffering. Although Bobbie is horrified at the implications of his friend's request, he respects David's desire to be freed from his tormenting pain.
His merciful deed done, Bobbie tells us that he lost his youthful innocence forever on the day he pushed his friend to his death. Bobbie learns that unintentional mistakes can result in gruesome consequences that precipitate horrific moral choices.