Fry goes to his night job at the Head Museum where he feeds the preserved heads of the Presidents of the United States. He invites the Planet Expresscrew to the museum for a party, where they become drunk and begin ingesting the preservative fluid inside the jars. Doing so causes them and everyone standing nearby to temporarily travel back in time to the eras each head originally came from. Professor Farnsworth reasons that this time travel effect is caused by the rare powdered crystalline opal used to make the fluid, which keeps the heads alive in a temporal bubble. After learning from George Washington's head that one of his own ancestors, David Farnsworth, was one of American history's most nefarious traitors during the American Revolutionary War, Professor Farnsworth becomes determined to salvage his family's reputation. He dumps the world's entire powdered opal supply into Washington's jar and licks his head, transporting himself, Bender, Fry and Leela back to colonial-era New York.
The four learn from the Continental Congress that David Farnsworth works at Benjamin Franklin's print shop in Philadelphia, where David is forging counterfeit money that would destroy the country's economy should it enter circulation. Though they do not find David at the shop, they discover a fake Massachusetts halfpenny and determine he has gone to Paul Revere's silver shop in Boston. They capture David just as Revere begins his ride to alert Lexington of the imminent British attack that would start the American Revolution. However, Fry takes one of the two lanterns hung at the Old North Church to burn the forged money, causing Revere to wrongly warn of the British attack "by land" rather than "by sea."
The four are suddenly sent back to 3011 and find that history has been altered: Great Britain has won the Revolutionary War and taken over all of North America, turning it into "West Britannia." In this alternate timeline, David Farnsworth killed George Washington and was rewarded with a dukedom, making Professor Farnsworth a noble landowner and consort of the Queen of England. Having depleted the world's crystalline opal supply, Hubert despairs that there is no way to travel to the past to fix their mistake until he notices an opal on the queen's crown. After stealing and crushing it, the four are able to use the preserved head of David Farnsworth to return to colonial times and restore the timeline. Once they return to 3011, everything is restored as it was before history was first altered, with one change: hanging in the Head Museum is a flag displaying Bender and a colonial spelling of his catchphrase "Bite my fhiny metal aff" in place of the Gadsden flag.