Family Guy star Mila will play a young woman who arrives in Ashton's character's world just as he's about to propose to someone else.
Mila and Ashton met in 1998 on the set of comedy series That '70s Show, where they played Jackie and Kelso.
Romance blossomed a couple of years ago, after Ashton separated from wife Demi Moore and Mila ended her eight-year relationship with Macaulay Culkin.
It was reported at the end of February that Ashton had proposed to Mila, a story that was fuelled when Mila was snapped wearing a diamond ring on her engagement finger.
Ashton and Mila have always kept their romance away from the spotlight so it's no surprise neither has commented on the reports.
Whatever is happening, however, it's clear they are as smitten as ever, as they were snapped courtside at a basketball game in LA last night whispering in each other's ears and sharing a kiss.
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