Following the conclusion of Debt of Honor, Jack Ryan is sworn in as President of the United States minutes after becoming Vice President. With nearly every executive, legislative, and judicial figure deceased, Ryan is left to represent the United States by himself. Ryan deals with various hardships and crises, from reconstituting the House and the Senate; to a challenge on his legitimacy by former vice president Ed Kealty; to a brewing war in the Middle East.
When the Iraqi president is assassinated by an Iranian agent, the Ayatollah Mahmoud Haji Daryaei takes advantage of the power vacuum by launching an unopposed invasion of Iraq. The Ayatollah unites the two countries, calling it the "United Islamic Republic", or "UIR". With Indian and Chinese assistance, the UIR makes a bid for superpower status by attacking Saudi Arabia. Following a series of Iranian-backed terrorist attacks, including the release of an Ebola strain, the UIR declares war on both Saudi Arabia and Kuwait. Meanwhile, China "accidentally" shoots down a Taiwanese airliner.
As a result of the Ebola attack, Ryan declares martial law and enforces travel restrictions in an effort to contain the virus. However, the attack becomes only a limited success for the UIR, since the virus is so deadly that it cannot spread effectively. The tide soon turns against the UIR, with its forces being defeated against the combined firepower of the U.S., Saudi Arabia, and Kuwait. President Ryan sends Ding Chavez and John Clark into the UIR to assassinate Daryaei. After showing the destruction of Daryaei's residence during a televised press conference, Ryan threatens to launch a tactical nuclear strike on Tehran unless those responsible for the attacks are extradited to the U.S. to face charges.
Kealty's challenge to President Ryan's legitimacy fails in court. In the aftermath of the crisis, appreciation of the unelected president grows. Then in answering a question from a reporter in the White House press room, Ryan says he will seek election to the office of President of the United States.