Earth astrophysicists discovered radio transmissions from a planet 130 light-years away at about the same time aliens on that planet discovered Earth transmissions. After much effort, humans and the pig-like Fofogo learned each other's languages and began joint research into starships. Eventually, something usable (at significantly below light speed) was developed along with cryogenic hibernation techniques. An expedition was launched by each planet to meet at the halfway point for a summit. Earth's expedition was led by Louise Donner while the Fofogo were led by Pahk Tschapp. The two leaders met at a space station constructed jointly between the two starships.
It did not go well. Donner was overcome with Tschapp's appealing body odor, bludgeoned him to death, and began eating him. The Fofogo abandoned space travel and cut off all contact with other species.
While Turtledove has not explicitly stated such, "Donner Summit" appears to be a satirical pastiche of Damon Knight's "To Serve Man". The porcine appearance of the aliens is consistent with Knight's original 1950 story, rather than the 1962 televised adaptation.
The title "Donner" is presumably a reference to the Donner Party, a group of American pioneers who migrated to California in a wagon train from the Midwest. During the winter of 1846-7, the migrants were stranded in the snowbound Sierra Nevada, surviving by eating the bodies of those who had succumbed to starvation and sickness. "Donner Party" has become pop cultural shorthand for cannibalism.