The prisoner, on the contrary, was at that time at a lonely spot called Marston's Spinney, where he had been summoned by an anonymous note, couched in blackmailing terms, and threatening to reveal certain matters to his wife unless he complied with its demands. 
The prisoner reluctantly went to Marston's Spinney because he was being blackmailed. 
The prisoner wholehearted went to Marston's Spinney to confront the anonymous letter-sender. 