Most critics feel that Christie had exhausted herself by the 1960s--and they frequently point to THIRD GIRL as a case in point. But although the novel is very much a "minor" effort by the Queen of Crime, it is nonetheless well written and often amusing, a quick and entertaining read.
The plot concerns three young women who share a London apartment at the height of that city's "swinging sixties"--when drugs, pop art, and wild clothing fashions are at their height. And one of the young women, Norma, thinks she just may have committed a murder. This nagging worry drives her to Hercule Poirot's door, but she both intrigues and annoys Poirot when she rejects him as too old to be of any use. Fortunately, Poirot is not over the hill quite yet, and with the aid of novelist Mrs. Oliver he begins to unravel the mystery of a murder that may or may not have happened.
This is one of Christie's more farfetched novels, a tale that relies on multiple impersonations and a solution that hinges on the recognition of several unlikely coincidences; even so, the book still has considerable interest in the way Christie looks upon the counterculture of the era--and takes a leap away from her typically "cozy" mysteries to make certain aspects of the counterculture key to the motivation of her typically complex plot. Dated? In the sense that it regards an era long past and therefore alien to most moderns, yes... but one would do better to regard it as a period piece. Final word: unlikely to become a favorite, but worth reading once.
GFT, Amazon Reviewer