Thomas Nash was a hospital patient who had once been on S.H.I.E.L.D's gifted Index as a self-proclaimed psychic. After failing to demonstrate his supposed abilities, he was taken off of the Index. Nash was later murdered by Agent Grant Ward.
Nash was a successful bonds trader in Canada until the Canadian government recruited him for a secret project with the goal of finding gifted people like Nash who could control other people's behavior. Nash claimed, however, he couldn't control human behavior, but occasionally he was able to predict it. It was at this point S.H.I.E.L.D. added him the Index.
However, four years prior to "End Of The Beginning", Nash was in a car crash which left him severely brain damaged and completely catatonic. S.H.I.E.L.D. then crossed Nash off the Index thinking that he couldn't use his powers, if he even had them, if he was catatonic.
In "End Of The Beginning", Coulson theorizes that the Clairvoyant isn't on the Index, but rather a person who was rejected from it. Agents May and Blake head to an assisted living facility in Macon, Georgia where Nash has been listed as a patient since his accident. However, after arriving, they're immediately attacked by Deathlok and find out later Nash was never at the facility.
Fortunately, Agent Blake shot Deathlok with a special tracer bullet which they used to trace him to an abandoned race horse track in Pensacola, Florida. A S.H.I.E.L.D. team storms the building and begin a shootout with Deathlok. After losing Deathlok in the underground sewers, Coulson and Garret find Nash in the buildings basement in a wheel chair, watching several different televisions, communicating with them through a computer monitor.
Nash then immediately surrenders and explains he wanted to be caught so he could see Agent Coulson face to face. Furthermore, Nash taunts the agents, particularly Coulson, about them knowing "the truth", giving them multiple veiled threats to Skye's life and warning them about a "force beyond their comprehension". Supposedly pushed to the limit, but in reality working under orders, Ward shoots and kills Nash in cold blood before S.H.I.E.L.D could take him into custody.
- After his death, Coulson and Skye find files that coincided with all the information that the Clairvoyant had and are suspicious about the Clairvoyant's lack of knowledge outside of S.H.I.E.L.D files and also about the briefness of the investigation. They then realize that Nash was not the real Clairvoyant and was merely put in place as a decoy.