Treating this episode as a revamp to the series, I wasn't particularly bothered by a lack of explanation as to how Prue died and how Phoebe escaped the underworld. Truth is, fans knew of the onset squabbling and falling out, and the writers did a commendable job at attempting to move on without Shannon.
All that said and done, Part One is an impressive Charmed episode. Despite some obstacles, Brad Kern manages to produce a compelling script, aided by series best performances by Holly and Alyssa, respectively. Rose McGowan, too, fits in nicely here, with a mix of all three former leads and a unique touch of her own, Paige's debut is solid.
There are plenty of fantastic little moments Charmed rarely does so well tucked away inside this first part. Piper gets the bulk of them and I especially enjoyed her attacking Leo, asking him why he didn't do anything to save Prue. It's a good way of tackling the subject of why Prue died without digging too deep into the already convoluted reasoning behind it.
Shax is back, which is always good. He's probably one of the best villains the show has ever had. It's a pity he looses some of his mystic appeal, with Leo jumping on him hoping for a jockey back. It would have been good to keep him around for the 2nd part, as the Source has yet to become to nefarious villain due to obvious casting issues, and he mucks up the dire second half completely.
Overall, it's the better half of this two hour special and probably one of the best Charmed episodes of the season. There's an emotional hook for fans reeling from the loss of Prue, there's quite a few impressive action set pieces, and the script has plenty of lighthearted moments in an otherwise dreary, full on cry-fest.