In the first paragraph of Chapter 1, what situation occurs, the resolution of which is the main thrust of the tale?

In the first paragraph of Chapter 1, Frank finds out that his wife has left with their three children. He learns this through a letter she sends him by messenger. Frank arrives home early from work and the messenger has not left yet to deliver the letter, but is having tea in the kitchen. After he reads the letter slowly, Frank informs his staff that Nellie has left taking their children with her, but he does not know for how long.