The epidemic strikes everyone who has passed through puberty. Abby Leigh is looking forward to watching the moon turn purple. For months, astronomers have been predicting that Earth will pass through the tail of a comet. They say that people will see colorful sunsets and, best of all, a purple moon. But nobody has predicted the lightning-fast epidemic that sweeps across the planet on the night of the purple moon. The comet brings space dust with it that contains germs that attack human hormones. Older teens and adults die within hours of exposure. On a small island off the coast of Maine, Abby must help her brother and baby sister survive in this new world, but all the while she has a ticking time bomb inside of her -- adolescence.