Charlie Chan at Treasure Island

Charlie and Jimmy Chan travel by plane to San Francisco. Paul Essex, Charlie's novelist friend, dies aboard the aircraft after receiving a radiogram warning him not to ignore 'Zodiac'; his briefcase disappears, drawing the investigation into focus.

Charlie meets Deputy Police Chief J.J. Kilvaine and re-encounters reporter Peter Lewis. They discuss the suspicious death of Essex and start tying Dr. Zodiac to the case, while Jimmy begins tailing Gregory.

Charlie, Rhadini, and Lewis visit Dr. Zodiac's home, where he conducts an eerie séance. The group senses Zodiac may be fraud, and the encounter reveals how deeply he is entwined with SF society.

Eve Cairo, Lewis's fiancée, has been meeting with Dr. Zodiac, angering Lewis and tying Eve to the suspect. This personal connection raises the stakes for Charlie as he pursues the truth.

Kilvaine reveals that Essex was poisoned, though suicide cannot be ruled out. The revelation broadens the motive and points to a blackmail scheme that might be connected to Zodiac.

Jimmy spends the afternoon shadowing insurance executive Thomas Gregory, whom he suspects stole Essex's briefcase. He discovers Essex's manuscript hidden in Gregory's hotel room, increasing Gregory's involvement in the mystery.

That night, Charlie attends Rhadini's magic show at the Golden Gate International Exposition on Treasure Island. He mingles with Eve Cairo, Bessie Sibley, and Rhadini's jealous wife Myra. During Eve's telepathy act, someone thinking about murder causes a knife to be thrown at Charlie.

After the show, Charlie, followed by Rhadini and Lewis, breaks into Dr. Zodiac's home. They find Jimmy already there and uncover Zodiac's vast files used to extort the rich. Abdul, the Turkish houseman, escapes; the burglars burn Zodiac's office to protect the innocent.

Essex's manuscript is a fictional account of Zodiac's blackmail scheme, and the next morning Charlie discovers that the last page revealing the killer is missing. Charlie suspects Gregory and questions his alibi as Gregory denies involvement.

Kilvaine reveals that Gregory is actually Stewart Salsbury, an insurance company executive plotting behind the scenes. The reveal shifts suspicion and tightens the circle around the financial motive behind the crimes.

At Kilvaine's suggestion, a public test of Dr Zodiac's claimed abilities is arranged. A note is found on the back of the missing manuscript page, mentioning a pygmy arrow and hinting at the hidden display in the Temple of Magic.

That night, Dr Zodiac appears during Rhadini's show and is killed with a pygmy arrow. It is revealed that Dr Zodiac is Abdul, and that a hidden mechanism and a brittle bow were used to simulate the murder while Abdul himself is killed.

To flush out the killer, a murder is re-enacted with Peter Lewis standing in for Zodiac. The secret of Rhadini's levitation is revealed, and Rhadini is stabbed in the aisle during the act, heightening the drama.

Myra uses the sphinx, a hidden elevator coffin, to slip from the stage to backstage. Eve taps into Charlie's thoughts to read the killer's motives, but Zodiac's mind interferes. She ultimately reads Zodiac's mind and discovers the killer's identity as Dr Zodiac, just as Jimmy spots a pistol and stops a fatal shot.

In the climax, Rhadini is revealed to be Dr Zodiac. He had used a spring-trigger wand to fire the arrow that killed Abdul and stabbed himself to divert suspicion, ending the murderous ruse and clearing the innocent.