Mortimer plays an actress named Emily -- possibly a version of herself? the show doesn't make that clear in its premiere episode -- whose best friend, Dolly (Wells), gets dumped by her boyfriend and visits Em in L.A. where Em hires Dolly to be her personal assistant. Dolly previously worked in a London restaurant.
Dolly knows nothing about being a personal assistant, which is where hilarity is supposed to ensue. I think.
Most of the comedy seems to come from Emily's Hollywood blinders. Emily returns from a dinner after Dolly has been locked out of the house -- in a bikini -- complaining about what a terrible experience she had at a posh Hollywood shindig, oblivious to shivering Dolly, who had wrapped herself in an outdoor lounger cushion for warmth.
"The insensitivity of these [expletives]," Emily rages.
In another scene, the girls discuss the desire to have ice cream but no one makes a move: Dolly waits for Emily, as her host, to get it; Emily waits for Dolly, as her assistant, to get it. That gag was kind of a nice, funny moment but the humor is very much of the low-simmer variety.
Much of the comedy comes out of that type of awkwardness but "Doll & Em" is less a comedy-of-the-uncomfortable series (a la a female "Curb Your Enthusiasm") and more just kind of slow and boring.