When someone takes a pen or pencil and adds a comment, an underlining or a question mark to the pages a book - is it cause for celebration or condemnation? And if the person with pencil in hand happens to be famous, should that colour our view? These are the questions Simon Armitage - himself a lapsed margin scribbler - wrestles with as he sets off to tell the story of marginalia. He begins with his own books, tracing his past through the comments he made as a much younger man first embarking on the world of poetry. He sees examples from some who've become regarded as masters of the art, such as William Blake and Samuel Coleridge, and hears how an Elizabethan torturer and a 20th century dictator had a great fondness for marginalia in common. He also heads to an antiquarian book fair, where he stumbles across an early volume of Dylan Thomas poetry, containing the poet's own drunken scrawl which has put the volume's price up to a handsome £8,000. Chiefly though this is a personal meditation for Armitage, bringing him into contact with a younger version of himself and reminding him of the profound power of physical books and the markings placed in them across decades and sometimes centuries.