Cardiac injuries after penetrating chest trauma are uncommon but potentially life threatening; these injuries can remain occult during the early stage because of the cardiac reserve of youthful physiology and may present at a later stage as the initial damage progresses or compensatory mechanisms fail. We report a case of unusual penetrating cardiac trauma from a posterior intercostal stab wound that affected both the interatrial septum and the tricuspid valve, leading to a stormy presentation as a result of the development of an acute right-to-left shunt followed by a successful surgical repair.