Improbable? You bet! Half the fun of this book is the B-movie TechniColor melodrama that Durrell lays on with trowel in hand and tongue almost certainly in cheek. What saves this from being a Grisham-style potboiler (fun in its own way) is the suspicion that Durrell doesn't believe in the plot any more than you do: the whole show's just a vehicle for his ideas. The shifting combination of doubles that each character pairs with in the story's weird geometry hints at the concept that everyone in the novel might just be an aspect of the same binary consciousness. The narrative style too--which loops and reloops languidly from past to present, then swoops in a flash to a climax, like one of Benedicta's falcons--tips you off that the workings of memory and the subjective sense of time it brings to our fragile notion of reality are as much a concern to Durrell as any of the events that unfold in his exotic & highly artificial world.
By today's standards, Durrell's prose is more than a little purple; that his women are basically walking dummies and his Orient the perverse, decadent hothouse of the British imperialist also marks "Tunc" as the relic of another era. But if you liked the "Alexandria Quartet" and want to recapture some of the magic, this book should fill a few pleasant afternoons.
P.S. "Tunc" forms a pair with "Nunquam"--both part of Durrell's "Revolt of Aphrodite" series--and each makes more sense if you read it in conjunction with the other.