U.S. Marine Colonel Lyle MacArthur "Mac" Casey discovers an apparent plan by Chairman of the Joint Chiefs of Staff General R. Pendleton Lloyd and Secretary of Defense Charles Potter to remove President William Foster from office and replace him with Vice President Walter Kelly, who they feel would be more willing to do their bidding.
Casey and Foster desperately seek evidence before the coup occurs. They encounter a setback when Attorney General Arthur Daniels (who has told Potter about his misgivings about the constitutionality of the plan) is murdered. Casey contacts long time acquaintance Betsy Corcoran, who is Foster's Chief of Staff, and is able to secretly bring his concerns to Foster.
With only a few days left until the coup, Casey receives unexpected support from "Jake", a Russian operative able to supply the kind of satellite and photographic proof that Foster cannot risk requesting of anyone in the compromised US intelligence community. When Jake provides access to Sarah McCann, secretary to the murdered Daniels and secretly a Russian agent, Lloyd's operatives assassinate her.
When all else fails, with troops deployed in 12 major cities and Lloyd and Potter in the Oval Office demanding Foster's resignation, Casey advises that he will confess to being part of their conspiracy (having uncovered enough information to sound legitimate in his so-called confession), denying the true conspirators any chance of claiming that they have acted within the Constitution.
Lloyd resigns, and Foster tells Potter that he and that other conspirators will simply resign in stages over the coming year, in order to prevent knowledge of, and a panic regarding, their treasonous acts.
Ken Parish Perkins of the Chicago Tribune stated "Messiness aside, "The Enemy Within" offers more good acting than most TV movies, particularly from ever-consistent Whitaker, Robards and Waterston."[6]
