They put their lives on hold – leaving Hannah's bedroom untouched – as they single-mindedly sought out her killer.
When he ran away to India, they followed. When he battled extradition, they returned to Delhi again and again.
When Maninder Pal Singh Kohli finally appeared in the dock at Winchester Crown Court, they were seated above him in the public gallery.
Now, finally, half a decade spent pursuing the truth has come to an end. Neither has worked in that time and they – along with their other daughter Sarah, now 19 – will have to pick up the pieces of their lives.
On March 14, 2003, Mrs Foster was cooking dinner at home in Portswood, Southampton. Her husband Trevor, who worked for gas exploration company BG Group, was playing badminton.
The cardiac nurse laughed as Hannah played the fool, dancing to music before going out to meet a friend.
But when Mrs Foster got up at 5am the next day to go to work, she noticed her daughter's bed lay untouched and she had received no word from Hannah.
"That wasn't Hannah, you could trust her 100 per cent," the 52-year-old said. "Call it a mum thing but I feared the worst."
These fears were realised two days later, when Hannah's body was found dumped in brambles in West End, Southampton, a few miles from home.
As the horror of what had happened sank in, the family was inundated with messages of support.
Mrs Foster said: "I never realised how many lives she touched in just 17 years. An old man stopped me in the street and told me how she always used to stop and pet his dog. He was in tears about her.
"Every parent thinks their child is special but Hannah was special."
Within two weeks, Hampshire Police had identified their main suspect, Indian-born Kohli, who had fled the country.
The Fosters instantly made public appeals, asking for information that could lead to his whereabouts.
Six months after her daughter's death, Mrs Foster was hit with the news that she had breast cancer.
She said simply: "I think of what Hannah had to endure. It doesn't come close."
A year on from the killing, Kohli seemed to have disappeared without trace.
Hannah's parents decided to go to India themselves, to make people realise how important it was to find their daughter's killer.
Armed with posters of Hannah – written in English and Punjabi - and a five million rupee (£60,000) reward, the teenager's picture soon adorned newspapers nationwide.
Within four days, Kohli had been arrested.
On hearing the news, Mrs Foster said: "Trevor and I were in floods of tears. We were just in each other's arms hugging each other shouting 'we've got him, we've got him'.
"A man is in custody and I feel that Hannah would be very proud of her mum and dad for what they have done to get justice for her.
"Whenever my courage starts to fail me I feel her there saying 'don't give up, go girl go'."
Mr Foster, now 58, added: "Our visit exceeded even our wildest dreams."
But joy soon turned to dismay, because although Kohli made a public confession and pledged to admit his crime, he changed his mind and fought extradition.
The Fosters returned to India in February 2006 to try and speed up proceedings, but Kohli used the trip to delay proceedings further, claiming the couple was interfering with judicial process.
But Mrs Foster said they had to go back: "Our lives are on hold there's nothing to look forward to.
"I have nightmares both day and night in which I replay over and over again what I imagine Hannah going through. In the dreams, I am watching it happen but can't do anything to stop it."
By 2007, the Fosters were increasingly worried that their daughter's killer would never return to the UK.
Mrs Foster said: "When we saw Hannah's body in the morgue we made a promise to her that we would get justice. As her parents, this is the last thing we can do for her.
"I want lovely memories of my daughter to be there but all we will talk about until justice is done is the night she died. Our lives are frozen in time."
Mr Foster, who ended up taking early retirement, went on: "We cannot move on because our every focus is getting justice.
"In the four years since Hannah died other people have new priorities. But for us nothing has changed. We haven't given up, but it is so hard for us."
They did not have long to wait. In July last year, extradition was approved and Kohli was back in Britain within days.
Mrs Foster said: "We have been so focused on getting this to happen. At times we thought it would never happen."
The family were regularly in court – only leaving to avoid the most harrowing evidence – and were there to see their final battle brought to a close.
Maninder Pal Singh Kohli, a 41-year-old sandwich delivery driver, was convicted today at Winchester Crown Court in Hampshire, United Kingdom for the murder and rape of teenager Hannah Foster. The 17-year-old girl was walking home from a night out in Southampton on March 14, 2003, but never returned home. Her body was found two days later, on Allington Lane in West End, a village just outside the city.
Map showing location of Hampshire
According to court records, Hannah had spent the evening with friends just a short distance from the family home in Portswood and was snatched by Kohli just yards away from her house. She was able to call 999, the emergency services line, hoping that the operator would hear what was happening. However, with nobody actually on the line an automatic system led the call to be terminated. After dumping her body, Kohli went home to his wife and children, before fleeing to India four days later.
Kohli spent sixteen months running from authorities, during which he remarried. Eventually he was arrested in a joint operation between Hampshire Constabulary and Indian police. Shortly thereafter, he gave, then retracted, a televised confession of the murder.

Kohli remained in custody in India while Hilary and Trevor Foster, the victim's parents, campaigned for his extradition to the UK to face trial. The campaign was eventually successful and Kohli arrived in Britain in 2007. He has been sentenced to life imprisonment and will serve a minimum of 24 years.

"Today we are feeling an overwhelming sense of relief at the verdict in this trial," said Trevor Foster in a statement to the press. "We have long realised that Kohli is a cold, calculating and totally ruthless man."