Maj. John Sheppard, Lt. Aiden Ford, Dr. Rodney McKay and Teyla have been visiting the world of Hoff, which, technologically, is about a century behind Earth. McKay smirks at their crude attempts at geothermal energy. But nobody laughs when Druhin, the Hoffan chancellor, shows them a prototype of a drug his people have been working on for 150 years - an inoculation against the Wraith.
The Hoffans - who have preserved and worked on this research for generations, as the survivors of each Wraith culling pass it along to the next generation - aren't expecting the Wraith to return to ravage their planet for another five decades. It's Sheppard's sad duty to inform them that he and the Atlantis team accidentally awakened one of the Wraith hives when first arriving in the Pegasus galaxy, and that the Wraith could arrive at any time.
The Hoffans are enraged at first, but calm down after Sheppard offers Atlantis' advanced technology and the help of Dr. Carson Beckett in perfecting the drug. Beckett, who hates being demolecularized for stargate travel, isn't thrilled about being volunteered. Dr. Elizabeth Weir, for her part, gives Sheppard a reluctant "go", on the condition that he back out gracefully if the drug turns out to be a pipe dream.
Beckett, however, thinks the drug has possibilities. It's based on a unique protein that one of the Hoffans possessed 150 years ago which rendered the person immune to Wraith attack. Beckett tests the protein cells, which were preserved all these years, on the Wraith arm that Sheppard brought back from the Atlantis team's first encounter with the vampiric aliens. Beckett and the Hoffans want to take the experiment to the next level - testing the protein's effectiveness against the cells of the Wraith whom Atlantis recently took prisoner. Sheppard calls him "Steve" since the creature hasn't given out his name or any other information in the weeks he's been starving in Atantis' holding cell, with no humans on which to feed.
Weir okays the test. Steve is sedated using a Wraith weapon. The drug proves viable. Now Beckett just needs to synthesize the protein into a viable serum. While Beckett and Perna, a beautiful Hoffan scientist, work on perfecting it, Sheppard interrogates Steve.
Beckett and Perna come up with a working drug prototype, but Beckett is alarmed that Chancellor Druhin and Perna want to test it on Steve. This means a human would have to be injected with the serum and put in Steve's cell to see if it will actually prevent Steve from feeding. The Hoffans have terminally ill citizens willing to risk their lives to test the drug. Weir is wary of using a human guinea pig or their prisoner for scientific tests, but as Sheppard points out, Steve is dying anyway. Weir green-lights the experiment.
Steve is brought to Hoff, having agreed to give Sheppard vital intelligence about the Wraith in exchange for being allowed to eat. Steve tells Sheppard that there are many hive-ships like the one Team Atlantis awakened. They are now all operational and will exhaust their own feeding grounds, gathering strength - and then come for Atlantis. The Hoffan subject, Merell, is brought in and Steve attempts to feed - but after touching the man, immediately pulls away.
Druhin is delighted and informs Beckett he's contacted his governing council and requested approval for the drug to be mass-produced and distributed. But Beckett is alarmed at the chancellor's rashness and insists more testing needs to be done. This could have been a fluke, or could work only on dying individuals, or resulted from any number of things.
Shortly afterward, Steve dies - but not of starvation. Beckett's postmortem shows that the chemical the Wraith secrete into a human to prepare them for life-draining mixed and mutated with the serum to create a poison that shut down all Steve's organs simultaneously. So the serum not only protects humans, but kills the Wraith as well.
This is good news to Druhin, but Teyla and Sheppard warn him that as soon as the Wraith discover this, they will wipe out the Hoffans and any civilization who has contact with them. The Wraith aren't the type to go away quietly - they will attack en masse and in fury. Even if they cannot feed, they can kill, and millions will die. Sheppard refuses to help the Hoffans if they won't agree to work on the drug with Atlantis until it is defensive in nature only. But it's too late. The Council has approved mass production of the serum and the inoculations of the population have begun.
Then the unthinkable happens: Half the people inoculated die, including Merell and, to Beckett's horror, Perna. As inoculations continue the ratio holds, and Druhin, the Council and 96 percent of the Hoffan people feel that the drug's 50-percent mortality rate is acceptable if it means defeating the Wraith. Druhin wants to discuss distributing the drug to the rest of the Pegasus galaxy, but Sheppard wants no part of the plan and prepares to leave with his team.
Beckett is disgusted at the Hoffans' disrespect for life, and for the first time he disagrees with the immortal words of Winston Churchill during WWII: "Victory at all costs."