It's a two-part, and impressively elaborate parody of the movie musical Mamma Mia!, done as a scathing yet affectionate homage, (Quite a trick to pull off.) But what's notable is the cast. It stars Jennifer Saunders as Meryl Streep and unites Saunders' two comedy lives -- her original partner Dawn French, and her Absolutely Fabulous partner Joanna Lumley. The latter two play Julie Waters and Christine Baran...well, no, actually Joanna Lumley doesn't. You'll see. And then toss in for good measure Sienna Miller playing Amanda Seyfried as her daughter.
(Side note: in order to get a joke it helps knowing that the original film's director is Phyllida LLoyd.)