While Piper and Leo are away for a month seeing The Elders, Prue and Phoebe must face a series of demons known as Guardians, who help mortal murderers go free in exchange for the souls of their victims. After saving Darryl from a Guardian, Prue and Phoebe must testify in court, where it is later revealed that the judge and all the other people in the court room are demons. Phoebe falls in love with the District Attorney Cole Turner (Julian McMahon), however she does not know he is a demon sent by The Triad to kill the Charmed Ones. Meanwhile, Piper and Leo return and reveal that The Elders want them to end their relationship or else the sisters will get a new Whitelighter. However, Leo decides to propose to Piper in secret.

The sisters must help a young man who is turned into an owl by day and his girlfriend who is turned into a wolf by night. The two lovers have been cursed by the woman's warlock boss, who will lift the curse once she gives in to him or whenever there is a 'night within a day'. Meanwhile, Piper and Leo try to find a way to hide their wedding from The Elders. However, they eventually find out about the wedding and send Leo away during the ceremony. This episode was heavily inspired by the 1985 fantasy film Ladyhawke.

While Piper worries that Leo will never return, Prue and Phoebe try to save a little girl named Kate and a Fairy Princess named Thistle from evil trolls who lurk in the "tweens" and want to capture the princess. When Prue and Phoebe need Piper's help, her concern with Leo causes a spell to backfire, placing Kate in more danger. Meanwhile, Cole, who appears in his demonic form Belthazor, tries to steal the Book of Shadows.

On Halloween, a local coven leader from the 1600s named Eva (Clare Carey), summons the sisters back in time to save a witch named Charlotte (Sadie Stratton) and her soon-to-be born child from an evil witch named Ruth Cobb (Judy Geeson), who wants to raise the child evil. It is later revealed that the child in the womb is the sisters' ancestor Melinda Warren and Cole travels back in time to capture her at birth. Meanwhile, back in the present time, Leo and Darryl try to fight off two Grimlocks who were vanquished by the Charmed Ones once before, but were brought back by the power of Halloween.

Prue becomes obsessed with learning more about The Triad, and thinks that they are launching an attack when the manor is burglarized with her personal items missing. Piper cannot concentrate whenever she is with Leo in a romantic setting because she fears that The Elders are always watching them. Meanwhile, Cole discovers that a demon named Troxa is trying to kill the sisters.

Unwittingly following the signs laid out by Cole, Prue comes into contact with a seemingly innocent shut-in, Vinceres, who refuses to leave his apartment that is about to be demolished. The man is faced with unendurable pain caused by his ability of feeling all of the emotions of the city. After learning the man is an Empath, she casts a spell to relieve him of his power, but it is transferred to her. Prue is later told he is a demon given this power to serve as a curse, causing him the inability to prey on witches or innocents. She nearly goes insane from bearing this gift that she was not meant to have. But Prue eventually overcomes it, intensifying the pain Vinceres felt by finally astral-projecting into his body and destroying him.

In an attempt to destroy the Charmed Ones, Cole enlists the help of Andras, a demon who magnifies anger into rage. Meanwhile, when the sisters lose their magic after using their powers against each other, they are left defenseless against attacks from The Triad and Belthazor.

When the sisters discover that Cole is Belthazor (Michael Bailey Smith), Phoebe must make one of the toughest decisions of her life: either vanquish her boyfriend or keep one of the most dangerous demons known to exist alive.

Prue and Phoebe have their hands full when Piper is possessed by the evil spirit named Terra, on the day of her high school reunion.

The sisters trace a mysterious ice-cream van operator who abducts children, but the evil may not be who they initially suspect. Meanwhile, the girls' dad Victor comes for a visit.

The sisters must stop the warlock Eames (Steve Valentine) from executing his plan to steal specific powers from witches in his attempt to kill every Whitelighter in the world.

Prue discovers that her former college boyfriend is being led down the path to becoming a demon, and the sisters must risk their lives in an underworld wrestling ring to save his soul.

When a shapeshifting warlock tricks Prue into a shot-gun wedding in order to steal the Book of Shadows, Phoebe and Piper are lured to the dark side and begin to enjoy their new evil powers.

Phoebe feels the pain of an injured Native American named Bo trapped in a time loop of a deserted town she visits. Prue and Cole enter the time loop that is stuck in the mid-1800s to save Bo, and Phoebe, from death at the hands of a local thug, and break the time loop.

On the day of Piper and Leo's wedding, Prue's astrally-projected self adopts an uninhibited, wild personality that threatens to ruin the wedding and place her in jail for murder.

Prue, growing weary of her battles to save innocents and in fighting off the Angel of Death, is forced to turn her back on a dying soul, and must come to grips with her feelings towards fate and the death of her mother.

While the sisters must deal with the possibility of being separated as Piper and Leo plan to move out on their own, they recall a time before they knew they were the Charmed Ones, when the girls were coming to grips with the death of Grams and Phoebe's departure to New York.

A demon named Lucas wielding the Seven Deadly Sins uses his powers against The Charmed Ones. Phoebe becomes infected with lust, Prue with pride, Piper with gluttony, and Leo with sloth. It is revealed that those infected with one of the sins may self-destruct due to the nature of the infection.

Cole poses as Belthazor to infiltrate an internet organization run by evil, that plans on controlling the world's information flow on the internet. Meanwhile, the sisters must vanquish the warlock posing as the company's manager without exposing Cole's role in the situation.

The evil Brotherhood plots to sabotage Cole's relationship with Phoebe in an attempt to turn him back to evil. Meanwhile, Piper develops a new power, the ability to accelerate the molecules in an object.

Phoebe has a premonition of a young woman being killed by a demon in a phone booth whilst adding on to the Belthazor section of the Book of Shadows, thus causing her to think Cole is responsible. However, the sisters realize that this murder does not follow Cole's pattern. Piper learns that it is actually the demon on the reverse page that's responsible; a Banshee. The sisters then cast a spell to track down the banshee which turns Prue into a dog, but Prue gets hit by a car. The driver takes her home to care for her, thus taking her out of the action. When the banshee attacks the Halliwell manor she surprises Phoebe in the attic and, sensing her immense pain, which is what the banshee feeds on, over Cole's betrayal, screams at her. It is later discovered that although this scream would kill mortals, it has a very different effect on witches: it turns them into a banshee too. Piper blows the Banshee up but it is too late, Phoebe has already turned into a banshee. Leo warns Piper that if Phoebe kills, she will stay a banshee forever.

Prue and Piper bring a doctor to the manor, to protect him from a powerful demonic assassin named Shax, who was sent by The Source. When Shax breaks into the manor, he throws Prue and Piper through a wall, causing them to almost die, until Leo arrives with enough time to heal them. Phoebe uses a vanquishing spell on Shax but it does not kill him. While searching for Shax out in the streets, Prue and Piper are caught on live television using their powers, thus exposing themselves as witches. Phoebe goes to the Underworld to find Cole and while there, they are forced to strike a deal with Tempus to travel back in time to before it was revealed that magic existed. They travel back successfully but Phoebe, Cole, and Leo are now trapped in the Underworld, leaving Prue and Piper lying unconscious on the floor from when Shax blasted them in his initial attack.