A veteran detective presents practical instruction and candid advice for beginners, opening with qualifications such as courage, honesty, education, languages, observation, perseverance, disguise skills, judgment, caution, temper control, and common sense, and warns against becoming a hired spy. The text mixes procedural guidance on training, evidence weighing, surveillance, and disguise with illustrative anecdotes and first-person case sketches showing how novices learn on the job. It emphasizes ethical choices, stepwise learning through agencies or mentors, patience, and practice, and includes transcriber notes and commentary on acquiring opportunities and developing instincts necessary for effective investigative work.