 The Enterprise investigates an epidemic progressing from planet to planet, spreading mass hysteria. When they witness a one man-ship from Denova commit suicide by setting course to the sun uttering as last words  finally free , a party beamed down finds what Jim feared: the plague has stricken and killed his brother, research biologist Sam Kirk, whose kid Peter is brought aboard. Then they find the invaders have the form of a flying jelly-fish with a terrible sting injecting multiple alien tissue beyond surgical therapy; even Vulcan physiology doesn t prevent it causing fatal madness but Spock s mental force eventually overcomes it- do the million colonists on the planet stand any chance, or should they be sacrificed to save billions in the entire galaxy? Spock beams down again and collects a specimen, to search for a cure or weapon...