The central twist of "Terminator 2: Judgment Day" is the role reversal of the T-800. In the first film, the T-800 was the antagonist, a relentless killing machine. In the sequel, an identical T-800 is sent back as a protector for John Connor. The opening act of the film plays on the audience's expectation, with both the T-800 and the T-1000 searching for John, leaving the audience to wonder which is the hero and which is the villain until their first confrontation.
A major plot point is the revelation that Cyberdyne Systems, the company that will create Skynet, is reverse-engineering the technology from the remains of the first Terminator. This creates a bootstrap paradox, where Skynet is essentially responsible for its own creation. This discovery motivates the main characters to shift their strategy from simply surviving to actively trying to prevent Judgment Day by destroying Cyberdyne's research.
The film's climax sees the destruction of both the T-1000 and the T-800. The T-1000 is defeated by being knocked into a vat of molten steel. In a heart-wrenching turn, the heroic T-800 insists that it too must be destroyed in the same manner to prevent its technology from being used to create Skynet in the future. Despite John's tearful protests, the T-800 sacrifices itself, giving a final thumbs-up as it is consumed by the molten steel. The film ends on a hopeful note, with Sarah Connor's voiceover stating that the future is not set and that if a machine can learn the value of human life, perhaps humanity can as well.