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The Secret of Secrets is a mystery thriller novel by American author Dan Brown and the sixth installment in his Robert Langdon series, following Origin. The book was released on September 9, 2025.

As of January 2026, the book has been featured on New York Times' best-seller list for 17 weeks in a row.

Robert Langdon is in Prague with his girlfriend, noetic scientist Katherine Solomon. He attends her lecture on human consciousness. The morning after the lecture, he sees a strangely dressed woman on Charles Bridge. The encounter reminds him of an ominous dream Katherine had during the night, culminating in him evacuating their hotel after suspecting an imminent explosion.

The threat turns out to be false, and Langdon is detained by ÚZSI captain Janáček and his protégé and nephew, lieutenant Pavel, as there actually was a bomb that was disarmed earlier that morning. Janáček thinks Langdon and Solomon staged the incident as publicity for her upcoming book. With US Embassy attache Michael Harris forcing the officers to behave, Langdon tries to prove his and Solomon's innocence by leading them to Crucifix Bastion, where Solomon had a planned meeting with fellow scientist Brigita Gessner. However, the facility appears deserted. Langdon discovers a secret entrance to the lab which he uses to escape Pavel. In Gessner's lab, Langdon finds her corpse and Sasha Vesna, who Gessner brought from a mental health institute in Russia to be her assistant after treating her severe epilepsy with a brain implant.

Unknown to Langdon, Katherine is targeted by a secret group called In-Q-Tel, which is working on an underground facility in Prague codenamed "Threshold", in which Vesna, Gessner, and the U.S. Ambassador Heide Nagel are also involved, and led by former CIA agent Everett Finch. Because Katherine's research into human consciousness overlaps with Threshold's operations, and her outspoken views against ever weaponizing such research, Finch had her upcoming book closely monitored. Katherine was invited to lecture in Prague by Gessner as a pretense to do reconnaissance on the nature of her book and get her to sign a non-disclosure agreement to view Gessner's lab that would also let Finch prevent publication of her book if necessary, though Langdon unknowingly thwarted this plan. After viewing a copy of the book provided by hackers, Finch sees it as a serious threat to Threshold and tries to prevent its publication. To this end, he has hackers infiltrate the publishing house that planned to release it and destroy all digital copies and backups. Two operatives then kidnap editor Jonas Faukman to obtain and destroy the only physical copy and interrogate him to identify any remaining copies before allowing him to escape to track his calls, but are ultimately caught and arrested. The woman Langdon saw on Charles Bridge and the bomb threat was also arranged by Finch to confuse and distract Langdon. Simultaneously, Threshold is targeted by a mysterious person known only as the Golěm who self-identifies as Sasha's protector, and murders both Gessner for experimenting on her and Harris for spying on her for Nagel, then goes after Threshold itself.