On March 19, thousands of passengers disembarked from the Ruby Princess cruise ship in Sydney harbour. Their 'luxury' cruise holiday had been cut short after authorities announced cruise ships would be banned from Australian ports as part of measures to stop the spread of coronavirus. Far from protecting people, the release of the Ruby Princess’s passengers instead triggered a public health emergency with the cruise ship now named as the single largest source of Australia’s coronavirus infections. On Monday Four Corners investigates how the Ruby Princess became an incubator for infection, leaving its passengers and staff dangerously exposed.