Do some people inexplicably catch fire and burn to ash within minutes? Well, spontaneous self-combustion may be a well-attested phenomenon--but Shaw uses it here as a springboard for hackneyed aliens-among-us folderol. Covering such a combustion case, middle-aged journalist Ray Jerome's suspicions focus on a mysterious country doctor. While tracking down the doctor, another man goes up in flames before Jerome's eyes; and the doctor turns out to be a human-alien with advanced mental powers, one of a community living underground on Mercury. Weak in physical science, the benevolent Mercurians are gradually transferring their minds to Earth by occupying the bodies of the terminally ill: the Earth minds end up in Mercurian bodies, while it's a simple matter for the Mercurians to cure their acquired bodies of disease; people burn up, it seems, if the mental focus slips during the exchange, exposing the Earth body to solar heat. And there are various complications, adding bulk if not plausibility. With an unconvincing New Hampshire setting (Shaw is far too British): predictable, workaday, often rather silly stuff--from an author who can do much better (Orbitsville; Other Days, Other Eyes).