Federal Marshal William O'Niel is assigned to a tour of duty at the titanium ore mining outpost Con-Am 27, operated by the company Conglomerates Amalgamated on the Jupiter moon of Io (moon). Conditions on Io are difficult: gravity is 1/6 that of Earth's with no breathable atmosphere, spacesuits are cumbersome and miners carry their own air supply. Shifts are long but significant bonuses are paid. The general manager, Mark Sheppard, boasts that productivity has broken all records since he took over. Carol, O'Niel's wife, feels she cannot raise their son Paul on Io and leaves with their child to the Jupiter space station to await a shuttle back to Earth. Tarlow, a miner, suffers an attack of stimulant psychosishe sees spiders and rips open his spacesuitresulting in death by explosive decompression. Cane, another miner, enters an elevator without his spacesuit during another psychotic episode and dies from decompression. With the reluctant assistance of Dr. Lazarus, O'Niel investigates the deaths.