The Thirty-Nine Steps

John Buchan, a Scot, wrote the adventure book The Thirty-Nine Steps.
Richard Hannay, a mining engineer who had previously worked in Rhodesia, returns to London in 1914. Franklin P. Scudder, a neighbor, claims to be investigating a group of German spies known as the Black Stone who are attempting to steal Britain's naval defense blueprints in preparation for war. Hannay leaves his flat while it is being watched, feeling as though he now has to thwart the plan. He poses as the milkman. Hannay boards an eastbound train but deviates from the path by getting off between stops. He ultimately comes upon an inn and convinces the proprietor to let him remain the night.
Fortunately, when on the moor, he comes upon a road mechanic who is severely intoxicated. The grateful employee is sent home for the day when Hannay offers to take over for him. Unfortunately, it turns out that person is Hannay's lethal foe and the head of the spy ring. Unexpectedly, he gets a call from London informing him that Karolides has been killed. The next morning, when Hannay and Sir Walter return to London, they clear his record with Scotland Yard, which then releases him.