From What Far Star

When Harry Sellars, a Security Officer, first became suspicious of one of the physicists working at the Space-ship Development Center, he did not imagine that he would end up by helping to win Earth's greatest battle. At first, when he saw a man walk through a solid wall, he thought that he was going mad. But his later investigations proved his sanity quite conclusively. <I>Something</I> had walked through walls, all right--but certainly not a man. Sellars' adventures in tracking down the alien and learning of its purpose on Earth, form the basis of this intriguing science-fiction story.