Fun All About Eve type story.
Gloria Munson (Linda Kelsey) imposes herself on Mary claiming to be the sister of Mary's classmate from high school (who Mary can't even remember) and soon asks about jobs in TV. Mary helpfully invites Gloria for a studio tour. Gloria quickly ingratiates herself with Ted and Sue Ann, is promptly a stand-in on Sue Ann's show, then with help of her biggest 'fan', station manager Ed Schroeder (Ron Rifkin), sees her role on the show rapidly expand, while Sue Ann fumes.
The episode neatly sets-up the MTM regulars attending Rhoda's wedding in the spin-off Rhoda. Ted Knight was conspicuously absent from that episode. Not sure why.