Donner
Donner
[Three people stand outside a restaurant; Megan, a man with some hair and another shorter person, probably a woman. There is a sign above the door which says "Joe's" (presumably the name of the restaurant) and a menu next to it. Outside the door, there is a maître d' with a cap behind a lectern. There is a sign on the lectern which says "Eat in".] maître d': Donner, party of four? Man: Actually, never mind. Megan: We're full.

The title text explains to the reader that the Donner party was a group of pioneers who set out west along a new route that was supposed to be easier to travel, but ultimately proved slow and treacherous. They became trapped in the Sierra Nevada mountains and many died. Low on food, many of the pioneers resorted to cannibalism, eating the bodies of party members who had already died, it is believed.
In this comic, three Donner party members arrive at Joe's restaurant, where they have ordered a table for four, as given by the fact that the maître d' knows they are the Donner party and calls for a party of four. The three decline the table since they are already full. This suggests that they just have eaten the fourth member (unknown to us) of their party after they placed the order for a table at the restaurant, but before they strolled over to it anyway.
Of course, since they are not in a survival situation in this strip, cannibalism would be completely unnecessary. However, it may be possible that they are suffering from Wendigo Psychosis . Alternatively, the Donner Dinner Party may have resorted to cannibalism because it took so long to be seated at the restaurant.
In the early days of xkcd (and days when Wikipedia was not quite in the mainstream consciousness), Randall didn't trust people to understand his comics or his references, and often explained the joke in the title text like here and for instance also in 38: Apple Jacks .
[Three people stand outside a restaurant; Megan, a man with some hair and another shorter person, probably a woman. There is a sign above the door which says "Joe's" (presumably the name of the restaurant) and a menu next to it. Outside the door, there is a maître d' with a cap behind a lectern. There is a sign on the lectern which says "Eat in".] maître d': Donner, party of four? Man: Actually, never mind. Megan: We're full.
