This is based on a true story that happened many, many years ago in Scotland. One day Mr Clark was walking home with a smile on his face. He was carrying something very valuable in his hand: tickets for a long, long journey.
This is the beginning of a true story https://www.test-english.com/grammar-points/b1/past-simple-past-continuous-past-perfect/3/
What I don't understand is the use of progressive tenses in this little paragraph. For me both actions will be completed, nothing arrived when he was walking and carrying and it is not said that both actions (carrying and walking ) are happening now, only one day . In fact I understand that both actions are the background of the story and that they are happening now so progressive tense is needed but that seems strange because one day does not mean now for me