Charlie Chan at Treasure Island

Paul Essex was killed because he was investigating Dr. Zodiac's blackmail scheme. He received a radiogram warning him not to ignore Zodiac, but the warning came too late, Essex was poisoned aboard the plane. His briefcase, which contained his manuscript about Zodiac's crimes, was stolen, likely to prevent the evidence from reaching authorities.

Earlier in the flight, Charlie learns that Essex had been corresponding with his friend about the mysterious Zodiac figure.

Dr. Zodiac is revealed to be Fred Rhadini, the magician. He created the entire Zodiac persona as a fraud to blackmail wealthy San Francisco socialites. The real Dr. Zodiac was Abdul, the Turkish houseman, who was impersonating Rhadini's false identity. Rhadini orchestrated the entire scheme to extort money from his clients.

Rhadini discusses the Zodiac case with Charlie early on and suggests Zodiac might be exploiting fears of the wealthy.

The killer used a spring-loaded magic wand to fire the pygmy arrow. During the show at the Temple of Magic, Rhadini used this mechanism to fire the arrow that killed Abdul (who was posing as Dr. Zodiac). The weapon was disguised as part of his magic props, making it appear to be a supernatural killing when it was actually a concealed mechanical device.

A note mentioning the pygmy arrow is found on the back of the missing manuscript page.

Deputy Police Chief Kilvaine organized a controlled re-enactment of Zodiac's murder to flush out the real killer. By having Peter Lewis stand in for Zodiac during the levitation trick, investigators could observe who might attempt another killing or react suspiciously, helping them identify the killer's method and motive.

Kilvaine suggests a public test of Zodiac's claimed abilities earlier in the investigation.

Essex had written a fictional novel about Zodiac's blackmail scheme, essentially documenting the real crimes. The manuscript's final page, which would have revealed the killer's identity, was missing, stolen to protect the culprit. This manuscript served as a road map for Charlie's investigation and contained crucial clues, including a note about the pygmy arrow.

Jimmy discovers the manuscript hidden in Gregory's hotel room.

Rhadini stabbed himself in the aisle to divert suspicion away from himself as the real killer. By appearing to be a victim rather than the perpetrator, he attempted to establish an alibi and throw Charlie Chan off his trail. This self-inflicted wound was part of his elaborate plan to escape accountability for killing Abdul.

During the re-enactment, Rhadini is stabbed in the aisle, which surprises everyone.

The sphinx was a hidden elevator coffin apparatus used by Myra Rhadini to move between the stage and backstage dressing rooms. During the chaos of the investigation, this mechanical device allowed hidden movement around the theater, though Myra herself was not the killer. It added a physical dimension to the already complex stage setting.

Myra uses the sphinx to move between areas during the magic show.

Thomas Gregory is actually Stewart Salsbury, an insurance company executive. He posed as Gregory to investigate (and potentially profit from) the insurance implications of the Zodiac blackmail scheme. His true identity was revealed by Kilvaine at police headquarters, shifting suspicion toward the financial motive behind the crimes.

Jimmy tails Gregory and discovers he's hiding something in his hotel room.

Eve Cairo is a genuine mind-reader who attempted to identify the killer by reading thoughts in the room. During the climax, her psychic attempt revealed a chain of potential motives involving several suspects. Her readings, combined with Charlie's deduction, helped expose that Rhadini was the true killer, though Zodiac's mind initially interfered with her abilities.

Eve's telepathy act during the magic show draws attention to the power of mind-reading.

Abdul was killed because he was impersonating Dr. Zodiac, a role created by the real Rhadini. When Rhadini needed to eliminate his decoy to cover his tracks and silence someone who knew too much, he used the pygmy arrow during the magic show. Abdul's death was meant to look like the supernatural Zodiac murder, but it was actually a staged execution.

Abdul escapes with a holster during the break-in at Zodiac's home, indicating he's involved.

Dr. Zodiac was a fraud who claimed psychic and supernatural abilities to intimidate wealthy victims. His séances and mind-reading acts were elaborate performances designed to make people believe he had genuine powers, allowing him to blackmail them more effectively. The truth was that his entire identity was a fiction created by Rhadini for the extortion scheme.

Rhadini discusses how Zodiac might be exploiting the fears of wealthy clients.

Essex's briefcase disappeared during the chaos of his death aboard the plane. It was stolen, likely by someone connected to the Zodiac operation, because it contained his manuscript documenting the blackmail scheme. Jimmy later discovers the manuscript hidden in Thomas Gregory's hotel room, proving Gregory's involvement in covering up the crimes.

The briefcase vanishes in the chaos, noted as a small but telling detail.