The story of "Terminator 4" is set after "Judgment Day" (July 25, 2004). The main character, Marcus Wright, was a death-row inmate in 2003 who signed a consent form before his execution allowing his body to be used for scientific research. He awakens in 2018 and makes his way to Connor's base, where he discovers that he has been transformed into a hybrid of human and machine (a Terminator). Connor doesn't trust him, but only he can help Connor infiltrate the Skynet core to rescue Connor's father, Kyle Reese - if Kyle dies, Connor will never be born.
Marcus and Connor reach an agreement and enter the Skynet core one after another. Inside, Marcus discovers that the so-called scientific research was actually a trap; Skynet had long planned to modify humans to infiltrate the human resistance, and Marcus's role was to bring Connor and Kyle to Skynet so they could be killed. Realizing the truth, Marcus insists that he is human, not a machine, and helps Connor rescue the imprisoned humans. Together, they defeat a T-800, and in the end, Marcus even donates his heart to save a severely injured Connor.