ON THE PLEASANT SHORE OF THE FRENCH RIVIERA, about half way between Marseilles and the Italian border, stands a large, proud, rose-colored hotel. Deferential palms cool its flushed façade, and before it stretches a short dazzling beach. Lately it has become a summer resort of notable and fashionable people; a decade ago it was almost deserted after its English clientele went north in April. Now, many bungalows cluster near it, but when this story begins only the cupolas of a dozen old villas rotted like water lilies among the massed pines between Gausse’s Hôtel des Étrangers and Cannes, five miles away.