The book introduced Scobie Malone, an Australian cop who became a long-running series character. In the film, he's played by Rod Taylor, who was probably at the height of his fame in the late 60s. Scobie is summoned to Sydney to go on a special mission to London - to bring back the Australian High Commissioner, who is suspected of having murdered his first wife. The order comes, incidentally, from Russell Napier, the veteran cop from the Scotland Yard TV series, whom I hadn't realised was indeed Australian.
In London, Scobie is persuaded by the High Commissioner - Christopher Plummer, at his most charismatic - that he needs a few days' grace before returning home, in order to conduct delicate (if rather vague) negotiations about world peace. Plummer's character proves to be a target for assassination, and Scobie finds himself confronted by a host of sinister and mysterious characters ranging from Dahlia Lavi, Burt Kwouk, Clive Revill, Lee Montague, and Derren Nesbitt.
Overall, this is a competent rather than memorable thriller. The script-writer, Wilfred Greatorex, was a familiar name on the credits of TV shows at the time, and the ingredients are good, but I have never been a huge fan of Rod Taylor, who was, let's face it, no Sean Connery. Personally I'd have liked the storyline to focus more on Plummer's character. I found him more complex and more interesting than Scobie.