Danny has just broken up with Margaret because he felt that she was not emotionally invested in their relationship, the specific word he using to describe her being "empty". That words sends Margaret on a journey of self-reflection, most specifically reviewing her past. Her thoughts head to her tenth birthday party, which she wants to redo for her upcoming birthday, she inviting the attendees from that gathering, the majority with the exception of her mother who she has not seen or been in touch with in years. But it is a specific incident from that tenth which may provide Margaret with a sense of why she is the way she is.
—Huggo