Once every fifty-two years Arcadia's six erratic moons come together in a constellation that plays havoc with the ecological balance of the planet. As a marine biologist at Riverside Research Centre, Mark Swindon is chiefly concerned about the effect of catastrophic tides on his precious fish pens. Then, without warning, a wave of seemingly motiveless violence sweeps through the normally sleepy colony - and Mark too feels himself drawn against his will into a mysterious cycle of death and rebirth.