Jyn isn't in prison when the movie starts. Instead, she's out there living - probably pulling a heist or making a deal.

She's got a type: tall, dark, and brooding. So Cassian's sent to bring her in, by any means necessary, up to and including seducing her. Except this mission - Jyn - is messing with his conscience more than any before, which is ridiculous, really, considering he's been a spy, saboteur, assassin, and worse, and she's not squeaky-clean herself...

+ Han gets an appearance or mention as part of the criminal world.
++ Cassian helps Jyn with whatever she's working on.
+++ Cassian can't bring himself to sleep with Jyn until after he tells her the truth. But he is veeeery tempted.