The saddest story from the Olivia Newton-John biopic isn't even about Olivia Newton-John.

In the mid-1960s Olivia Newton-John and her close friend Pat Carroll were touring the UK as a duo and building a promising music career.

But there’s a reason why one girl ended up becoming a superstar, and the other never reached those heights – and it’s the perhaps the saddest part of Channel Seven’s biopic Hopelessly Devoted To You on Sunday night.

Olivia Newton-John cemented her place as an iconic musician and actress when she starred as Sandy in the film adaption in musical Grease in 1978.

But in the early 1960s she was a teen just trying to make it in the music industry after some early successes. She met her friend Pat Carroll while appearing on Go Show and won a talent contest on the TV programme Sing, Sing, Sing. 

Although initially reluctant to take advantage of the prize, which was a trip to the UK, she eventually went and began touring, but didn't really start to see success - and frequently wanted to return home to Australia - until her friend Pat moved there.

The pair then formed duo 'Pat and Olivia' in 1967 and toured nightclubs in the UK and Europe. In Hopelessly Devoted To You, we see Pat as the one who pushes Olivia on and encourages her to not be afraid to go after what you want.

But just when they were really starting to gain momentum, an unfortunate reality hit. Pat's visa expired. Reluctantly, the promising pop star returned home.

But Olivia, who was born in Britain, was able to stay.

In the movie event, a young Olivia (played by Morgan Griffin) thinks twice about returning to Australia, saying "I'm coming back with you. We'll get lots of work... We were doing this together."

But a selfless Pat (Georgia Flood) talks her out of it.

"What will you do? Every Australian TV show twice? Then what? Of course, you're staying here... Stay here and do it all for me... bloody stay here and make it happen."

4️⃣ days to go... Who is excited? ???????? @katejenko is PAT CARROLL @georgiaannflood is PAT CARROLL Don’t miss OLIVIA, on Sunday 8.30pm ????

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From there Olivia continued to pursue her music career and was recruited to the band Toomorrow, who released an album in 1970. Although it wasn't hugely successful, it led to Olivia releasing her first solo album  If Not For You in 1971, of which the title track was her first international hit.

And while there were setbacks and stumbles to follow, Olivia eventually enjoyed steady success, representing the UK in 1974's Eurovision along the way.

Pat's career followed a different path. After returning home, she married John Farrar, who was a member of the band, Strangers. Farrar is also the songwriter behind the Grease track Hopelessly Devoted To You.

Most of the songs she released in 1960s and 1970s didn't make it on to the music charts and her best-known singles were To the Sun and All Kinds of Everything.

She did return to the UK around 1970 after her husband left Strangers and later started singing back up on Olivia's songs during the 70s and 80s. In the mid-70s she and John relocated to the US, where they live now. The pair also welcomed to two sons, Sam and Max who have had music careers of their own.

And while we're sure Pat wouldn't have regretted any of this, we can't help but wonder if she wonders, from time to time, 'what if?'

You can watch Olivia Newton-John: Hopelessly Devoted To You Part 1 on 7Plus before Part 2 airs Sunday, 20 May at 8.30pm.