"It's about timeless relationships," said Grace, "your first kiss, for example."
The first season of the show was set in 1976. Grace said it started out as an examination of life for young people who were coming into their own as America celebrated its bicentennial.
"And we were celebrating what?" he said. "The end of Vietnam? Watergate?"
Grace, who plays the level-headed but socially awkward Eric Forman, wasn't even born until 1978. But he said the themes are so timeless he didn't even feel the need to research the period before starting to work on the show.
"What is there to research?" he said. "I was an awkward guy for 10 years."
Grace is adamant about another thing -- that the show is not about bellbottoms or '70s music.
"A lot of my friends wear bellbottoms now and listen to '70s rock," he said. "I don't know what kind of show it would be if it we were making comments about the styles and the music."
Still, when the show filmed its 100th episode recently, the guest star was Roger Daltry -- lead singer of The Who, and a '70s rock icon if there ever was one.
"It's great to have Roger Daltry in our rock opera," he said. "We decided to do a show that's for the fans who have been with us for four years."
Grace said landing Daltry for the show closed the book on a disappointing turn of events early in the life of the series.
"We wanted to use 'Baba O'Riley' (from The Who's 1971 album "Who's Next") as the theme music for the opening sequence," he said. "We got the big 'no.'"
Grace admitted to being star-struck when he was around Daltry.
"Oh, big time," he said. "I have like all of his albums in my car."
Even without Daltry physically present on the set, The Who are a continuing presence on the show.
"We have fan music magazines on the set with him on the cover," said Grace.
Grace said he and his colleagues couldn't believe the time had actually come for them to shoot their 100th episode. During their first season, he said, they all figured the show was going to be canceled.
Instead, it turned into a hit for Fox, which has picked up the show for a fifth and sixth season.
The longevity of the show reminded Grace of an observation he attributed to Candice Bergen, who starred in the CBS comedy "Murphy Brown" for 10 years: "You're going to look up and a decade will have gone by."
Kunis, 18, has appeared in features including "Get Over It" (2001) and "Krippendorf's Tribe" (1998).
Kutcher, 24, starred in "Texas Rangers" (2001) and "Dude, Where's My Car?" (2000).
Masterson, 26, starred in "Dracula 2000" (2000) and "The Faculty" (1998). He also had a recurring role in Cybill Shepard's CBS comedy "Cybill."
Apart from a cameo as himself in "Ocean's Eleven" last year, Grace has only appeared in one movie -- as a drugged-out preppie in "Traffic." He's been getting more offers for movies since the success of "That '70s Show" but his experience with Stephen Soderbergh's Oscar-nominated drug-war movie has left him less inclined to make movies just for the money.
"To me 'Traffic' is ... a testament to not doing bad stuff," he said. "It's hard to turn down big roles in those (teen coming-of-age) films, but basically they're not good films. They're made kind of on an assembly line."
Grace said those kinds of projects offer a chance to have some fun and make money, but not much else that appeals to him.
"At the end of the day I have to watch it," he said, "you know -- go to the premiere."
Grace said it requires a patient attitude to turn down work, but it's worth it.
"I would rather do a movie that has great intentions or an interesting idea that failed completely," he said, "because at least you tried."
Celebrity Families of 2014 [PHOTOS]