Plague Ship follows the adventures of Dane Thorson, a Cargo-master-apprentice on the Free Trader rocket ship the Solar Queen. Free Traders take on trading contracts on remote and recently discovered planets, which can be dangerous and unpredictable. The Solar Queen has recently obtained a valuable trading contract on the planet Sargol and are building a relationship with one of the races on the planet, the cat-like Salariki. The process goes slowly till the Salariki discover that the Solar Queen is carrying catnip and other plants from Terra that are unknown on Sargol. The traders exchange what little of the plants they have for the rare and valuable Koros stones and collect a native red-colored wood to exchange at home. A few days after leaving the planet, several members of the crew suffer from attacks, which start with severe headaches and end in a semi-coma state. Only 4 of the younger members of the crew are unaffected, including Dane Thorson. Upon exiting hyperspace on return to the vicinity of Terra, the crew discovers that they are pariah and have been declared a plague ship.