A defendant is acquitted of murder, then later confesses, or DNA proves the jury wrong. For centuries the law’s answer was that nothing could be done. This episode traces double jeopardy from Roman law through 800 years of English common law, where it shielded citizens from a state with unlimited time and money to retry them until it won. The price was accepted openly: better that some guilty people go free than that the government hold power to prosecute a person without end.

That bargain broke down in the face of real cases. In England, Billy Dunlop was acquitted of killing Julie Hogg in 1989, later admitted it, and could be convicted only of perjury until her mother, Ann Ming, campaigned for change. The Criminal Justice Act 2003 now allows a retrial for the gravest crimes on new and compelling evidence. The episode compares the answers reached in Australia, Germany, the Netherlands, Japan, India, and the United States, where separate governments can each prosecute the same act.

• In 2006 Dunlop pleaded guilty and became the first person convicted of murder after a previous acquittal for the same crime.
• In Australia’s R v Carroll, the High Court overturned a perjury conviction because it contradicted the man’s earlier acquittal for murdering a child.
• Under the American dual sovereignty doctrine, state and federal courts may both try the same act, a power used in the civil rights era after state juries acquitted.
• Timothy Hennis was acquitted of a triple murder in civilian court, then convicted of the same killings by a military court martial.
• A 1950 ruling in Japan treats a case as one continuous proceeding through every level of court, so prosecutors can appeal a not guilty verdict.