It seems one of those pesky little Replicators from Season 3's finale episode, "Nemesis," really survived the crashing of Thor's ship after all. Unfortunately for a Russian submarine, they happen to come by the lone techno-bug, who invades their ship and does mucho damage including, but not limited to, killing everyone onboard and eating the submarine's hull for replication.

Jack, Teal'c, and Sam survived the techno-bug menace that was the Replicators and had successfully gated off the ship before it exploded. Post-credit sequence, our three weary world-saviors ("This shouldn't get old, General," Jack tells Hammonds) have returned to Earth after a week of being stuck on the planet they had gated to. All 3 are anxious to come home, as Sam lets the SGC know that they've been dialing home every day for the last week. Their delay, of course, is due to Hammond having ordered the Beta Gate sealed up a while ago in Season 2 during some scuffles with a branch of boy scouts called N.I.D. Wonder what Sam and Jack were doing while off-world for a whole week?

After some genuinely shippy moments between Sam and Jack in Sam's lab, the ever-reliable, and sometimes-grating, Major Davies arrives with bad news about the survival of the lone Replicator bug and how it has turned the Russian submarine into an all-you-can-eat buffet. But while the Americans have intercepted the ailing Russian sub, they have no idea how to deal with the problem. Jack suggests nuking it, but Hammonds informs him of (an all-too convenient) strained relations between the U.S. and Russia that wouldn't be help by the nuking of a Russian sub by the U.S. Before our heroes can jump into action, Thor arrives via the Stargate in dramatic fashion to request aid in his planet's fight against the Replicators. Sam agrees to go, while Jack, Teal'c, and Daniel goes with Davies to do battle with that pesky Replicator onboard the Russian sub.

At the harbor where the Russian sub is being held, Jack and Teal'c and 2 other SGC commandos board the sub while Davies and Jack monitors them via helmet-cams. Once they have boarded the Russian sub, our favorite tag-team duo realizes the extent of the lone surviving Replicator's, well, replication: it has replicated a new army of Replicator bugs. Jack and Teal'c and the red shirts -- I mean, commandos -- fights their way back. Back at the command center, a decision is made to go back and waste the lone Replicator bug and then sink the sub, thereby killing the new Replicator bugs who are made out of the sub's hull, and therefore couldn't survive out in the open in the ocean the way their resilient mom had.

Meanwhile, Sam gets her first glimpse of the Asgard homeworld, and it's pretty darn impressive. Sam is also introduced to The O'Neill, the latest Asgardian ship built specifically to fight the Replicators, named of course after our favorite wise-cracking human. After being confronted by 3 Asgardian ships now under Replicator control, Sam comes up with a plan to destroy the pesky suckers before they can reach the Asgard homeworld and go Oprah on it. Her idea: use the O'Neill as bait to lure the bugs into hyperspace where they can't use their shields, then detonate the O'Neill and take out the bug ships along with it. Thor is hesitant to pursue the plan, but Sam correctly points out that this is precisely why she is here to begin with, to give the Asgardian a new perspective on their battle. It seems the Asgardian have forgotten how to fight dirty. Thor relents, and the plan works. The Replicators headed for Asgard is destroyed and Sam nearly snaps Thor's neck in a spontaneous bear hug.

Meanwhile, back on Earth, Jack and Teal'c have ventured back into the Russian sub to take out the "mother bug" so they can then destroy the sub and sink the rest of the "rust able" Replicator bugs. The mother bug gets hers, but Jack and Teal'c are trapped. Jack orders Daniel to give the go-ahead for an attack sub called the Dallas to waste the Russian sub. Daniel is rightfully hesitant to take out his best friends, but Jack takes the decision out of his hands and gives the order to Davies, who gives the go-ahead. As the missiles hit the Russian sub, Jack and Teal'c are in death's grasps, when -- you guessed it -- our heroes are whisked away via Asgardian transport beams just in the nick of time.

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