The thing about Richard Hannay, protagonist of The Thirty-Nine Steps, is that he's bored. "I was tired of seeing sights, and in less than a month I had had enough of restaurants and theatres and race-meetings" he tells us. In other words, he wants an adventure. If John Buchan hadn't made this so obvious in the first paragraphs of his book, it would be impossible to suspend belief and follow the frankly ludicrous story.
Hannay lets a stranger into his flat named Scudder, who spins him a tale of intrigue. Scudder is subsequently murdered and Hannay must go on the run. He's in danger of being the next victim of the murderers, has lied about Scudder to his man Paddock, might be accused of the murder by the police, and can't alert the authorities because that would play into the murderers' hands. So, he flees to Scotland.
The remainder of the story follows Hannay across the hills and glens, regularly trusting total strangers to offer him free bed and board. After all, he says, "A man of my sort, who has travelled about the world in rough places, gets on perfectly well with two classes, what you may call the upper and the lower."
Published in 1915, the book is of its time and a century on is unintentionally funny. According to its wikipedia page it was very popular with soldiers in the First World War. It's a thrilling read and in spite of, or more likely because of its fantastical story, it was "greatly appreciated in the midst of mud and rain and shells, and all that could make trench life depressing."