The plot was worthy of a Dan Brown thriller – two Charles Darwin manuscripts worth millions of pounds reported as stolen from Cambridge University library after being missing for two decades. The disappearance prompted a worldwide appeal with the help of the local police force and Interpol. Now, in a peculiar twist, the notebooks – one of which contains Darwin’s seminal 1837 Tree of Life Sketch – have been anonymously returned in a pink gift bag, with a typed note on an envelope wishing a happy Easter to the librarian. The bag was left on the floor of a public area of the library outside the librarian’s office on the fourth floor of the 17-storey building on 9 March, in an area not covered by CCTV. Who left them and where they had been remains a mystery. Dr Jessica Gardner, who became director of library services in 2017 and who reported the notebooks as stolen to police, described her joy at their return as “immense”. “My sense of relief at the notebooks’ safe return is profound and almost impossible to adequately express,” she said. “I, along with so many others, all across the world, was heartbroken to learn of their loss. “The notebooks can now retake their rightful place alongside the rest of the Darwin archive at Cambridge, at the heart of the nation’s cultural and scientific heritage, alongside the archives of Sir Isaac Newton and Prof Stephen Hawking.” It was back in 2001 that the notebooks, which represent some of Darwin’s first inklings of his radical theory of evolution by natural selection, were originally found to be missing. They had been removed from storage to be photographed, and work was recorded as completed in November 2000. But during a subsequent routine check made in January 2001, it was found they had not been returned to their proper place. At the time staff believed they may have been mis-shelved. A fingertip search of key areas in the library, which houses about 10m books, maps, manuscripts and other items, proved to be unfruitful, and the books were eventually reported as stolen to the Cambridge constabulary in 2020. The police force then launched an investigation and notified Interpol, with the university making a global appeal for information. Their return, almost a year and a half later, has both stupefied and delighted authorities. Prof Stephen J Toope, the vice-chancellor of the University of Cambridge, said he was “incredibly glad to hear of the notebooks’ safe return to their rightful home”. “Objects such as these are crucial for our understanding of not only the history of science but the history of humankind,” he said. The manuscripts were said to be in good condition and with no obvious signs of significant handling or damage sustained in the years since their disappearance. They were wrapped together with clingfilm inside their archive box. A plain brown envelope bore the printed message “Librarian/ Happy Easter/ X”. The library’s deputy director of Research Collections, Dr Mark Purcell, had previously said he was confident the manuscripts could not be sold on the open market and hoped for a similar outcome to that at London’s Lambeth Palace, where items were stolen after bombing during the second world war. “Forty-plus years later, quite literally as the consequence of a deathbed crisis of conscience, those items came to light and were returned to Lambeth,” he said. While there is no CCTV of the area where the manuscripts were returned, Gardner said entrances and exits to the building were covered, as were targeted areas such as strongrooms and specialist reading rooms. She said available footage had been handed to police, adding: “It really is a mystery. We don’t know how and we don’t know who.” Gardner said the library building had “transformed significantly” since then, with card-and-pin access to secure areas, an onsite security team, high-security strongrooms and additional CCTV. Further reviews were to come, she added. The notebooks are to go on public display from July as part of the library’s Darwin in Conversation exhibition. A Cambridgeshire constabulary spokesperson said: “Our investigation remains open and we are following up some lines of inquiry. We also renew our appeal for anyone with information about the case to contact us.”