Delia was in her boyfriend's bathroom when she found something that made her blood run cold.

When Delia Balmer met John Sweeney at a London pub in 1991, she thought she'd met someone special. He was a carpenter, artistic and charismatic. He wrote her poetry. He was attentive. He seemed perfect.

Little did she know that, at that point, he had not only killed his last girlfriend, but several other people.

Watch the trailer for Until I Kill You. Article continues after video. 

At first, John was everything Delia could have wanted in a partner. He showered her with attention, wrote her love poems, and seemed genuinely interested in building a future together.

Looking back, Delia would later understand that those romantic gestures — the poems, the constant attention, the intense declarations of love — weren't romance at all. They were the first steps in John's calculated path of control and abuse.

ITV's gripping drama Until I Kill You brings her haunting story to the screen, available for streaming on ABC iView in Australia.

Based on Balmer's 2017 memoir 'Living with a Serial Killer', the series stars Anna Maxwell Martin as the nurse who found herself trapped in a nightmare she never saw coming — dating a man who was a serial killer.

Delia met John Sweeney in 1991. Image: ITV

Within months, John's true nature began to emerge. The charming carpenter transformed into someone Delia barely recognised. He became possessive, controlling, and increasingly volatile. When she tried to create boundaries, he'd respond with rage. When she attempted to distance herself, he'd switch back to the charming man she'd first met.

But then she found something that made her blood run cold.

Hidden behind a bathroom panel was a suspicious bag. She knew exactly what it was: "A body disposal kit meant for me."

While she'd been living her life — going to work, coming home, cooking dinner — the man who claimed to love her had been methodically planning her murder.

"A body disposal kit meant for me." Image: ITV 

By 1994, the situation had become dire and, fearing for her own safety, Delia left. She changed the locks. She tried to keep herself safe.

Despite Delia's desperate warnings to police, John broke into her home. For four terrifying days, he held her hostage, binding her to a bed and threatening her with a gun. But it was what he confessed during those nightmare hours that would haunt her.

Between threats and rages, John revealed he had murdered his former girlfriend, 33-year-old Melissa Halstead. Her body had been found mutilated in a Rotterdam canal in 1990. He also claimed to have killed two men in Amsterdam.

When John finally left the flat, Delia did what she'd been waiting four days to do — she ran. She went straight to the police, reporting everything: the hostage situation, the threats, the confessions of murder.

John Sweeney was given four life sentences. Image: ITV 

The police arrested him. But not long after, they let him out on bail, and just one day later, Delia arrived home to find John waiting for her with an axe in hand.

In a horrifying attack, John severed one of her fingers — and might have done worse if not for the courage of a neighbour who heard the commotion and rushed to help.

Armed with a baseball bat, the neighbour struck John, interrupting the attack, saving Delia, but not before John was able to escape.

It wasn't until 2011 that John was finally brought to justice when police found him with an arsenal of four guns. A court handed down four life sentences for the attempted murder of Delia and weapons charges, as well as the murders of both Paula Fields and Melissa Halstead.

Delia's testimony proved crucial in securing these convictions.

According to the BBC, police now believe there could be more victims — three more women who vanished without a trace between 1970 and 1990.

Watch Until I Kill You on ABC iView. 

Feature Image: ITV.