During Fry’s night job he invites everyone to a party with all the president’s heads.
In an attempt to correct his family tree of one bad apple, the Professor travels back in to colonial America times with Fry, Leela and Bender.
They successfully thwart one of the Farnsworth’s but in doing so Fry used one of Paul Revere’s lamps and changed the course of history. Causing America to still be dependent on the British in the present.
The Professor goes back in time again and is able to correct everything.