There is a long running gag that if there is a new foal in town, Twilight will find some way to find it and take care of it. Thus, ponies have a habit of attempting to dump newborn foals on the young alicorn in the middle of the night.

Twilight is not too fond of having a nursery in her library, especially not for some other pony's foals. However, when Flash Sentry arrives with a basket, a blanket, and a desperate expression, the stage is set for a confrontation with the potential reality of the gag's implications.

Or a hasty conclusion that is as far off base as close as it is to making logical sense.