A demon is after Pandora's Box - if she can open it, she will release all the sorrows in the world.
Wyatt has entered the Terrible Twos, which means he's using his magic without permission. Piper is having trouble disciplining him, and she feels like Leo isn't any help.
Paige is plagued by a weird ringing in her ears. Leo tells her she's hearing the call for Whitelighters, and Paige isn't pleased. Leo gets Paige to talk to an Elder, who tells her the Whitelighter call is a gift and her destiny. When Paige says she'd rather suppress her Whitelighter side, Leo persuades her to go on a training mission - just go out and help a future Whitelighter, and see how it feels.
The demon Katya ambushes Nina, the Guardian of Pandora's Box. They fight, and the box disappears. Nina turns to the Charmed Ones for help. In every generation, a new Guardian is born. When the Box thought Nina wasn't going to survive the fight, it must have flashed over to the new Guardian. That Guardian is Hope, who finds the box in her dorm room. She cracks it open, and a buzzing swarm of sorrows escapes. Everyone is suddenly depressed, including Hope's friend Darcy, who she's talking to on the phone. Piper and Nina find Hope. Phoebe finds out Nina is dead and goes to warn Piper – but it's too late. Katya, who had shapeshifted to look like Nina, steals the box. Phoebe convinces Hope to come back to the manor with them. Hope is skeptical that magic even exists, until Wyatt orbs in front of her. Hope gets a call from Darcy and decides to leave. When Phoebe tries to stop her, Hope blasts her with some hand mojo. She seems to be coming into her Guardian powers.
Katya can't open the box - only Hope can do it. So Katya pretends to be Darcy and lures Hope to her lair. She tries to sweet-talk hope into opening the box - she doesn't want this responsibility! It's not fair that this is thrust upon her! - but when that doesn't work, she threatens to kill Darcy, who she's holding prisoner. Hope caves and opens the box. Piper and Phoebe collect Hope and the empty box - only Hope, as Guardian, can put the sorrows back in the box. Katya again disguises herself as Darcy and goes back to the manor with them.
Piper is trying to find her future Whitelighter when she gets stuck on an elevator with a cute little girl, a bickering couple and a sleazy guy. The ringing stops, so one of them must be her charge. Things get bad – the couple won't stop fighting, sleazy guy tries to hit on her, and the little girl is asthmatic and starts to have trouble breathing. Then they get worse – sleazy guy tries to climb through the hatch at the top to get the elevator started again, and get electrocuted. His heart stops. The little girl has an asthma attack. Paige takes charge – she gets the husband to help her with CPR while she performs mouth-to-mouth, and tells the wife to sing with the little girl to get her breathing back on track. Her strategies work, and everyone is saved.
Katya tries to convince Hope that she can't trust the sisters - hey, any one of them could be the demon! Hope is persuaded, and Phoebe tells her she has to make her own decision. She sends Piper and Katya downstairs while she has Hope learn how to contact the sisters if she needs them. When they leave the Manor, Katya returns to her original form and takes Hope down to her lair. She's going to kill Hope so the box will stay open. But surprise! It's not Hope at all, but Paige disguised as Hope, She vanquishes Katya and rescues the real Darcy. Hope embraces her destiny and manages to get the sorrows back into the box.
Paige meets with the Elder and admits that helping everyone in the elevator felt great. The Elder convinces her that she's a natural at Whitelighting, and that this experience proves she can be both a Charmed One and a Whitelighter. Paige agrees. But who on that elevator was the future Whitelighter she was supposed to help? You were, the Elder replies.