From beginning to end, the novel centers on identity and disguise. Syme’s first challenge is to discern between enemies and friends. The narrative describes Syme’s introduction to the anarchists and combines his observations of their external appearance—clothing, facial features, body type, and behavior—with his emotional reactions. They seem to be ordinary men, but with closer examination, Syme judges each man to be subtly “wrong.” The detectives initially represent evil and then are revealed as good. Lucian Gregory is the opposite. Syme believes he is harmless and mainly good, but he is exposed later as the ultimate evil.

Chesterton makes it clear identity cannot be determined by external characteristics alone; intangible attributes of personality are not as readily apparent. In a religious context, this concept represents the doctrine that “[…] the Lord seeth not as man seeth; for man looketh on the outward appearance, but the Lord looketh on the heart” (1 Samuel 16:7). When each man strips off his mask, their unseen identity is revealed. The professor is amiable and immediately bonds with Syme. Dr. Bull is young, mischievous, and delights that his deception was so convincing. The Marquis is amused as he taunts Syme to pull off his nose.