Sue's goal in this moving story is simple: she wants to keep her friend and roommate Johnsy alive. However, it is their heroic neighbor, Old Behrman, who makes Johnsy's survival a reality.
It is a cold November when the story gets going, and Johnsy has caught pneumonia. Johnsy's doctor tells Sue that her situation is critical, and the best thing Sue can do to help her is to make her think of something that she has to look forward to. Unfortunately, there is no man in Johnsy's life to give her the will to live, and odd as it may seem, the thing which ends up helping Johnsy to hold on is that the last autumn leaf on the tree outside her window was also holding on. When the leaf fell, Johnsy declared, she would die.
Enter our hero, the failed painter and heavy drinker, Behrman. When Sue explained that Johnsy had decided that her mortality was intrinsically linked to a leaf, Behrman sprung into action. Without either of the ladies knowing it, he went outside into the frigid winter conditions and painted a leaf onto the wall so that Johnsy would think her leaf was still holding on.
In the process, however, Behrman fell ill himself and ends up dying. In finally painting his masterpiece, Behrman died the hero that kept Johnsy alive and made Sue's ultimate goal a reality.
Sue's overriding goal in the story is to keep her good friend Johnsy alive by giving her something to live for. Struck down by a nasty bout of pneumonia, Johnsy has pretty much given up the ghost. As she's convinced that she'll be dead before long she, no longer has any interest in life, not even in painting, which is a very bad sign for an artist such as herself.
But Sue is absolutely determined to do whatever she can to keep Johnsy alive. She knows that the only thing that's keeping her friend going is the tree outside the window, whose leaves are slowly vanishing one by one. Johnsy has resolved that she will stay alive just so long as there are still leaves on that tree.
This gives Sue the idea to get an old alcoholic painter by the name of Buhrmann to paint a leaf on the wall. This will trick Johnsy into believing that the last leaf is still going strong. When the painted leaf is still there after a violent storm, Johnsy's spirits start to revive, and she slowly starts to recover from a disease which at one point had seemed fatal. Much to her delight Sue, has achieved her overriding goal.