The master of coin Lord Gyles is dead and Cersei is not happy about it, and blames Maester Pycelle. She gets Pycelle to tell her that he has been giving Margaery Moon Tea, which is a bit like the morning after pill and thus this is evidence that she has been treasonously having sex with her courtiers. Cersei, with Taena's help (and I'm not sure she should really be trusting Taena so much since it was not that long ago that Taena was spying on Cersei for Margaery) cooks up a complicated plot to prove that Margaery and her cousins are all having orgies all the time so that even Lord Tyrell won't be able to complain when his daughter is beheaded.
I thought this was all going to come about more easily a couple of Cersei chapters ago when Cersei was trying to get one of the Kettleblacks to seduce Margaery. I guess this proved too difficult. I strongly suspect that Margaery is not having orgies at all and the moon tea is for something else. Or else Pycelle is lying.
Qyburn and Cersei torture Margaery's singer Wat the Blue Bard into confessing what they need him to confess and giving them a list of names of people involved in the sex parties. It's all a bit vague how much of this Cersei believes and how much she knows she is faking it all just to get rid of Margaery. I'm not sure even Cersei knows. Nobody likes to think of themselves as the baddie so they will believe whatever fiction is necessary to think themselves good.
Cersei thinks this is all going to work and that Maggy's prophecies will be thwarted. She dreams about being locked up in the dungeon and Tyrion saying that all her children would be crowned and she would see them die. I think this was another part of the prophecy. She tells Taena also that she thinks Tyrion is meant to kill her.
Cersei tells Osney Kettleblack to go and see the new Great Septon and confess that he has been indulging in obscenities with the young queen. She says his punishment will be to go to the wall, where, if I remember rightly, he is to kill Jon Snow. He says he'll do it if she lets him commit obscenities with her, you know, just so he won't totally be lying to the Septon. Cersei goes along with it, because that's how she rolls.
Things are not looking good for Margaery, but I can see about a million ways this scheme can go wrong. Cersei has to trust quite a lot of untrustworthy people. The Tyrells themselves have successfully done quite a lot of skullduggery to get Maergery into this position. They must have felt somewhat secure in leaving her at King's Landing with only young cousins and Cersei. Don't they have any insurance policies?
