What is the plot of "The Last Leaf"?

In the short story “The Last Leaf,” by O. Henry, Sue and Johnsy met and decided to share a flat in May. In December, pneumonia started making the rounds in their neighborhood.  Johnsy got sick, and the Doctor told Sue that Johnsy had a 1 in 10 chance in surviving depending upon her attitude. Sue moved her painting supplies into Johnsy room to keep her company, and became puzzled when Johnsy started saying “ . . .twelve, eleven, ten . . .” She was counting leaves on the vine outside of their window, and she informed Sue that she expected to die when the last leaf fell.

Shortly thereafter, Sue was asking their old German neighbor Mr. Behrman to pose for her painting, and they got into a discussion about Johnsy, and she told Behrman that Johnsy expected to die when the last leaf fell from the vine. Behrman who was an artist who’d never painted a master piece agreed to sit for Sue, but when they look at the window at the vine they notice that the pounding rain and ice has knocked the last leaf off of the vine.  After painting Behrman, Sue falls asleep. The next day when Johnsy demands the blinds be raised so that she can see the vine, she notices there is still a leaf. The leaf stays and stays, and Johnsy decides she’ll survive. The next day they learn that Behrman has died of pneumonia, and Sue tells Johnsy the leaf Behrman painted outside the window was his life’s masterpiece.