In the 1970s, when a distant cousin dies unexpectedly, Mary Ellen and Charles Hawley agree to take his seven-year-old son Steven into their home. Steven's birth mother, a Polish immigrant, has schizophrenia and can't assume custody. But in a last traumatic visit, she convinces her son that a mysterious presence called the bicycle messenger has followed him into his new life.

Years later, Steven, a recovering alcoholic with a history of bipolar disorder, is working as a short-order cook. To Megan Helmsworth, a college student waiting tables, he is a sweet, doting boyfriend. But Steven becomes more and more preoccupied with his lost family of origin, and when a manic swing sends him on a quixotic journey to Poland, Megan seizes her chance to escape. In a well-meaning attempt to shelter her fragile son, Mary Ellen agrees to harbor a secret no mother should ever keep.

Told through the eyes of the three women who love Steven best-Mary Ellen, Megan, and his adoptive sister Margaret-The Bicycle Messenger is a story of generational trauma that stretches from the last days of the Kraków ghetto in 1942 all the way to 2016. But it is also a story of unexpected grace.