Simon Tam (better known in some circles as the old Zac Efron) and his sister River have been among Firefly's most intriguing characters. Not necessarily due to the chops of the actors that portray them but rather the recurring impact their histories have on the overarching plot of the show. While the back-stories for Firefly's supporting characters range from mysterious allusions (Shepard Book, for example) to entire episodes (Jaynestown, for example), the recurring impact of the Tams' suggests that theirs was meant to be one of the core drivers for the series.
In Ariel, the mystery behind what happened to River is deepened slightly as Simon employs the crew to help him sneak her into a state-of-the-art hospital on Ariel, a core planet in Alliance space. While there, Simon discovers that River has undergone multiple surgeries on her brain which, combined with the increasingly super-natural abilities of the young lady, is making this side story downright Fringe-ian. Where this will take us by the season's (and series') end is certainly addicting but I don't hold out hope for a satisfying conclusion given the premature nature of the show's demise.