It was a good episode. Phoebes new bf was destined to die, she kept saving him...cheating the Angel of Death. Cole (apparently) sends a messenger from the future to kill the bf, and thus save Phoebe from dying in 6 months time. When asked how he has the power to do this in the future, the messenger tells Cole that he (Cole) unites warlocks and demons and once again rules the underworld...only this time evil is the strongest it's ever been.
The warlock kills Paige and Phoebe by stripping the sisters powers. Luckily Piper and Leo figure out that there has to be a time ripple created when Phoebe saved her bf and that Piper can go back and allow him to die when he is destined to.
The future Piper finds the present Piper and tells her she has to stop Phoebe from saving him... or they will die. The Present Piper and the future Piper then watch as Phoebe watches her bf die. Tears all around.
When it's all said and done, Piper and apparently Leo remember everything the other sisters don't.