Two drivers crashed cars. Hanna (plaintiff) survived, but the negligent driver died. Hanna filed a lawsuit in federal court, based on diversity jurisdiction, against the other driver’s estate. As required under the Federal Rules of Civil Procedure, Hanna served a summons and complaint on the executor of the estate, Plumer (defendant). But Plumer was away from home, so the process-server handed the paperwork to Plumer’s wife. That approach was perfectly fine under the federal rules. But Hanna did not know that a substantive statute under Massachusetts estate laws provided that executors could not be liable unless they were served with court papers personally. Plumer moved for summary judgment. He admitted that the service satisfied the federal rules but raised the affirmative defense created by the Massachusetts statute, noting that he had not been served personally. Thus, Hanna’s service of process satisfied the federal procedural rules, but prevented the estate from being liable under Massachusetts’s substantive law. The district court judge dismissed the case. The First Circuit affirmed, and the United States Supreme Court granted certiorari.