As the title suggests, Cassian (and whoever)'s future child shows up. Their adult-Resistance-Officer future child. Obviously, said child can't actually tell them this, that'd probably break the universe or something. Hijinks ensue.

+ the child never actually lies to their parents (the best lies are the truth... from a certain point of view)
+ it's going well and then they get caught because of something incredibly simple (they swear exactly like Cassian, they know someone's birthday and they shouldn't, in the right light they look just like a parent, that kind of thing)
+ when the truth comes out Kay is appropriately horrified