Paradise Lost

Paradise Lost is the fifteenth episode of the sixth season of Stargate SG-1.

Plot
Jack O'Neill is visited by former Col. Harry Maybourne, who helped him in the past years when it comes to illegal NID operations. Maybourne tells him that he knows about the goal of Col. Simmons, when he tried to steal the Prometheus (to find a planet with ancient weapons) and that he knows the Stargate address of that planet.

O'Neill agrees and, together with the rest of SG-1, visits the planet without Maybourne. There they find out that the writings on the planet aren't of Ancient origin but instead belong to the Furlings, another one of the Four Great Races. However they don't know how to unlock the seal to get to those weapons, so they have to trust Maybourne, who later goes with them to the planet. Unbeknownst to SG-1, Maybourne has his own agenda in mind as he escapes from SG-1's charge and flees through the alien door, which turns out to be a transportation device, pursued by O'Neill. Carter and a team of scientists try to activate that device again, but fail. Searches come up empty; O'Neill and Maybourne are nowhere to be found.

The two adversaries are in the meantime spirited away to the planet's moon (though no one is yet aware of this) where Maybourne explains to O'Neill the significance of the moon, described in legend as an "alien utopia" where Maybourne had hoped to hide away from his checkered past on Earth.

As Carter begins a frantic project to reopen the alien portal, O'Neill and Maybourne discover an empty shell of a civilization with no survivors left to explain their demise. They both realize they may be stuck on this alien world indefinitely and begin to dig in for the long haul, which Maybourne doesn't make easy for Jack. After a week the scientists have to give up (much to Carter's protest) and even the Tok'ra don't find anyone on the planet.

O'Neill begins to uncover clues to the utopia's fate and notices increasingly paranoid behavior from Maybourne. He hears something at night, demands Jack's pistol and even runs away after a few days. Finally, O'Neill discovers dead Goa'uld hosts in some of the houses with a mysterious plant nearby that he identifies as the substance he saw Maybourne eating all throughout their stay on the alien moon. O'Neill returns to discover that Maybourne has taken weapons that O'Neill hid with the very intent of keeping them from the delusional Maybourne. However he also starts to become paranoid.

Finally, after a passionate attempt to recover O'Neill, Carter realizes that O'Neill and Maybourne were sent to the alien moon from MALP playback footage. By this time, however, Maybourne's paranoia has erupted into a fire-fight with O'Neill. When O'Neill tries to explain how the plant has affected Maybourne's mind, he is answered with more gunfire from Maybourne. The fight ends with O'Neill shooting Maybourne, effectively subduing the madman. O'Neill cares for the wounded Maybourne who now, with a cleared mind and injured body, can understand that O'Neill, no matter how much he may like to, isn't trying to kill him. Soon after, a Tok'ra ship, whose aid was enlisted by Carter, flies over their position, rescuing them from the empty paradise. Jack also says he will convince the Tok'ra to bring Maybourne to a nice planet on which he can retire.