Ellis Hock has spent most of his life behind the counter of his family's menswear store in Medford, Massachusetts.
When a new mobile phone betrays him - by revealing the flirty messages he has sent to female customers over the years - he forsakes his furious, now former, wife and appalling daughter by returning to Malawi where, almost 40 years before, he was at his happiest teaching English to the Sena tribe.
It is, of course, a huge mistake. Malabo, in the deep south of the country, surrounded on three sides by Mozambique, is dying. The church, school and clinic he helped to build are in ruins. The villagers are only interested in his money.
Stubborn, and not a little stupid, Hock stays, attended by Zizi, a beautiful teenage girl intended to snare him, and Snowdon, a leprous dwarf.
The heat and malaria soon take their toll. His only friends are the plentiful snakes who terrify the natives. It dawns on Hock that he will die unless he leaves Malabo but every attempt at escape ends in failure.
Paul Theroux describes the landscape - its scorched eucalyptus, mopane and sausage trees - with his trademark precision. The alien flora adds to Hock's sense of isolation and the way Theroux heightens the sense of menace is masterful.
Hock is held hostage in a village of Aids orphans, and refused help by a sinister charity. He finally realises the Sena were "changed, disillusioned, shabby, lazy, dependent, blaming, selfish; they were like most people . . . You could meet them almost anywhere".
Theroux has never written a better novel than The Lower River. Many will compare it to the works of Graham Greene but Hock does not undergo a crisis of faith. Conrad's Heart of Darkness and Waugh's A Handful of Dust are greater influences.
There is comedy among the horror. Hock's encounter with Fogwill, a British expat who burbles on about Major Moxon at the Gymkhana Club and Fred Horridge and his "horrible restaurant" is hilarious. Brits always seem to bring out Theroux's misanthropy.
However, it is the Sena, with their proverbs and secret dances, that dominate this African adventure - the moral of which is: "Never go back."