River Tam has been away from home for nearly four years, now. Her seventeenth birthday passed unremarked by their parents, who had at least made something resembling a nod to their daughter on her fourteenth and fifteenth. (She's in good hands, and they're loyal citizens. Worrying would be gauche, especially when their lives have gotten quite lucky ever since they signed their youngest daughter over to the government's academy).

She sent her older sister a letter on her own birthday, though - and on Ellie's birthday, too, and one responding to Ellie's letter to her. 

The food is still pretty boring here. Wish they'd let me put ghost pepper on it. Do you remember that - street stall I think it was, we split the curry, you turned bright red... Wish they had food like that here.

(Their parents have never let them eat street food.)

Have mom and dad let you get a puppy, yet? If they do, you should name her after me, so I can bite you in spirit.

(River had briefly wanted a puppy at six, but - the thing she'd spent most of her time bothering their parents for was a kitten.)

(Those aren't the only things wrong. It's been four years of slightly off - unverifiably wrong - memories and references, and woven through it...)

Do you remember when we went ice skating, that one time at great-grandma's? Do you think her house is still there? I'd like to go back, sometimes. It was a lot of fun. 

(River had fallen in, and her older sister had saved her life. River hadn't gone near water again for five years, and had still been avoiding ice when her sister last saw her.)

The probable location of the government academy is in the Herald system, in the same cluster as Ellie's school - two hops away, through wormholes placed conveniently closely to the planets. It's a quiet place, more tightly guarded than any prison Ellie has ever seen. It's plain. Sterile. Boring, but - threatening in its simplicity. Brutalism's descendant, grown up into a bully.

The security is thorough. 

Ellie is smart, though, and important people aren't questioned quite as much, and, well. There are quite a lot of planets containing quite a lot of politicians, some of them involved in projects like this. Some of them pretty similar in appearance to Ellie.