Then something happens.

Perhaps the husband is called away, it may have been to take part in the Great War.

The wife, whatever her tender solicitude for her absent partner, feels her solitude and is drawn nearer to friends, perhaps her husband's friends.

Some man among them becomes congenial to her.

There need be no conscious or overt love-making on either side, and if there were the wife's loyalty might be aroused and the friendship brought to an end.