For the first time since the Civil War, America has two sitting presidents - and the dangers and divisions run even deeper. "Father's Day" is a chilling thriller of constitutional crisis.

The reference is the Twenty-fifth Amendment: Presidential Vacancy, Disability, and Inability. The time bomb is section four: When and if a disabled president seeks to return to the Oval Office, there is a ruthless provision for the vice president, as acting president, to block him with a palace coup.