The literary device of the unknown man changing places with his famous double is difficult to carry off convincingly. In this book, the moral doubts are all on one side - the side of the ambitious, but poor and unknown man (Loder). The man who's place he takes (Chilcote) is a downward-spiralling morphine addict, who's conscience is conveniently already dead.
The bulk of the story turns on the rapid rise of Loder at what is presented as a turning point of extreme importance in British politics. His deep personal satisfaction at being, at last, important and a success is continually threatened by Chilcote's telegrams, which he must obey immediately to switch places, allowing the constantly worsening Chilcote to alienate his wife, whom Loder has begun to care for, and imperil his country.
The last part of the book is driven by Loder's final struggle with temptation - the temptation to let Chilcote follow his addiction to its natural conclusion, so he can permanently take his place.
I gave this book a two because ultimately, the plot was one that could not end for the protagonist both happily and honorably. Many torturous phrases are employed to justify an ending that, despite a thick coating of rationalization, is ugly and dishonorable. This made it, for me, an unsatisfying book.