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Finally, a musical that picks up where “Hair” left off.

“Heartbeats,” conceived by Amanda McBroom and Bill Castellino, is the warm, bittersweet story of marriage in the ’90s told through the eyes of Annie, who is about to celebrate her 40th birthday and 20th wedding anniversary on the same day.

Annie will be celebrating with her best friend, Lavergne, because her husband, Steve, will be watching a game with his buddy, Ted. McBroom’s lyrics deal with the sometimes comic vicissitudes of domestic life and the angst of Baby Boomers trapped in middle age.

“I want them back . . . the days when everything was possible. Before we settled down to comfortable despair,” one song goes.

It just opened at Marriott’s Lincolnshire Theatre, where it plays through Aug. 20.

“The story is sort of from my own life and the life of all my girlfriends,” says McBroom, an actress and singer who is well-known for penning the pop ballad “The Rose.”

“All my pals who hit 40 at the same time and went, `Oh, my God.’ It seems to be, like, a universal condition. . . . It does a big thing to people’s heads.

“We are indeed the Boomers and that was why I wanted to write it, too, because I was just watching it happen to everybody. `The days when everything was possible’–I must confess that’s one of my favorite lines. What a rude awakening!

“Like, I know I will probably never really speak good French. Am I sort of relieved about that? And I will never go into lion taming. You try to let go of it.”

“Heartbeats” had its world premiere in 1990 at the Old Globe Theatre in San Diego and played in about 10 theaters since then.

Performances are Wednesdays at 2 p.m. and 8 p.m.; Thursdays and Fridays at 8 p.m.; Saturdays at 5:30 p.m. and 9 p.m.; and Sundays at 2:30 p.m. and 7 p.m. Call 708-634-0200.