Series one explores the supernatural relationship between a fallen angel named Azazeal and a student called Cassie who is also a witch.

[1] From its outset, Hex appears to be the story of Cassie Hughes, a withdrawn young woman who enrolls at the school but cannot quite seem to fit into the social milieu.

The second series shifts the focus from the departing Cassie Hughes (Christina Cole) to a new heroine, Ella Dee (Laura Pyper).

Hex is set at a remote English school, Medenham Hall, which occupies the former manor house and grounds of the McBain estate.

Cassie, who raised herself in a single-parent household caring for her mentally unstable mother, has latent telekinetic, pyrokinetic and clairvoyant abilities that are awakened when she touches an antique vase that had been used in Voodoo rituals by the Medenham Witches.

Cassie has startling visions and dreams that she strives to interpret, and also struggles with controlling her growing telekinetic and pyrokinetic powers, which she is only able to summon in times of stress.

The school gains a new chaplain, Jez Heriot, who doesn't "wear the uniform" except when conducting services and also teaches Ethics.

Ella instantly takes charge of both Cassie and Thelma, and formulates a plan to kidnap Malachi and sacrifice him in the school chapel.

They are successful in grabbing the child, but Azazeal follows them and a series of alternately foolish and selfish decisions leads the girls inexorably to tragedy.

Azazeal appeals to Thelma's own selfish desires and convinces her to steal and give him the Stone of Belial, one of Ella's objects of power, in exchange for a last meeting with Cassie.

With the Stone in their possession, Azazeal and Ramiel conspire to drive Ella insane by forcing her to relive the 17th century witch trial in which she was found guilty and nearly executed.

Ramiel's scheming and manipulation leads to Ella's commitment in an insane asylum, where a triumphant Azazeal seemingly abandons her.

Ella kills Jez and makes it look like suicide, precipitated by the revelation that he was having an affair with a student, which deeply affects Roxanne, who felt responsible because she had seduced him.

Mephistopheles betrays the cause of evil by helping Ella save Leon, who he feels did not need to die, and is subsequently punished and has his eyes removed.

He eventually seduces the entire student body, creating an army of succubi and incubi that makes him too powerful for Ella to kill.

The only students who are not converted are Roxanne (who becomes a "pure soul" in her quest for forgiveness from God for seducing Jez and causing his apparent suicide), Tom (who is secretly gay, in love with Leon, and impossible to convert because Malachi cannot make Leon love him back), Thelma (because after Ella later kills Maya, he has no hold over her), and Ella herself, who had already been freed from Malachi's spell.

Jo Watkins, Cassie's former teacher and Azazeal's former lover, returns as Malachi's new emissary from the "higher powers" and restores her position as headmistress by framing David Tyrel for embezzlement of school funds.

Fooled by the false mark, Leon kills Tom, but by doing so, he achieves his greatest desire (not being inferior to Ella) and becomes an incubus of Malachi's.

He is locked in a cage and sedated, at which point Thelma saves him from Malachi by invoking his greatest fear (having his penis amputated).

Ella uses the Stone to travel back in time and attempts to kill Malachi while he is still a baby (at the end of Series 1), but she is attacked by Perie the Faerie and receives a mortal wound to the chest.

Recurring cast for Series Two included Stephen Wight as Felix, Jemima Abey as Alex, Sam Troughton as Jez Heriot/Ramiel, Ronan Vibert as Mephistopheles, Katrine De Candole as Perie the Faerie, Laura Donnelly as Maya Robertson, Leon Ford as Max Rosen, Grant Parsons as Dr. Surtees and Anatole Taubman as Raphael.