David is the new lodger who moves into the home of Polly Whittacker after her mother Ivy makes it clear to Reg, her husband, that he is not welcome there any more.
Naturally, Ivy chooses Polly's room to redecorate and turn into a lodger's bedroom, which banishes Polly to a tiny space not much bigger than a closet. And naturally Ivy begins to pay David a great deal of attention, as she had to Rex when their relationship was new. David ignores Polly at first, but eventually makes friends and takes advantage of that to ask her to take money down to the pub for him to give to a seedy-looking man standing outside it. Polly does so without knowing what that money is for, but Ivy finds the new closeness suspicious, and tells Polly she is going to have to go live with her father now.
This is a turning points in the novel, as it moves Polly in a new direction that changes many things for her.