New Delhi, Nov. 4 -- How a decade-old bribery plot on the island of Nauru let an Australian businessman build an $800 million empire, evade extradition, and poison democracy.

In the early hours of June 19, 2013, a panic-wracked Australian businessman boarded an international flight and held his breath. Hours earlier, Amit Gupta's Gold Coast home had been raided by the federal police.

Investigators had spent weeks tapping Gupta's phone, probing sensational allegations he had backed a political coup on the small Pacific Island of Nauru by bribing multiple politicians who had plotted to topple the government. The new government, Gupta hoped, would give him complete control over the island nation's lucrative mining rights, but an arrest now...