In 1912, 12-year-old immigrant William Alton's adoptive city offers a prize for the child who can improve sanitation by killing the most flies. William first sets out to win to avenge the sickness and deaths of his mother and sister, but competition with classmates almost makes him lose sight of the contest's real goal.

When twelve-year-old William Alton and his father sail from England for new opportunities on the Empress of Ireland, they leave behind the graves of his mother and baby sister. Will only hopes they will leave death and disease behind them, too. But a shipmate's baby falls ill and is forced into quarantine on Grosse Ile, and when they finally arrive in Hamilton - their new home - they discover that Will's Uncle Charlie is in the hospital. Why must everyone around him get sick and die?

A competition at Will's new school offers an answer...and a challenge. In 1912, cities around the world declare war on the fly, blaming it for the spread of the diseases - summer complaint, typhus, consumption and typhoid - that are wiping out families. To avenge his mother and his sister, Will throws himself into the local fly-killing contest.

Along the way, he makes enemies and friends: Fred Aitken, the wealthy classmate who hates losing; Fred's admirer, Ginny Malone, who often must skip school to look after sick siblings; and Rebecca, a doctor's daughter who hates poor sportsmanship worse than poverty and disease.

William uses ingenuity and determination to climb into the top ranks of fly-catchers, but along the way loses track of his real mission. Can he beat Fred without cheating? How much does it matter?