Mona is alive and has been held in A's dollhouse (the place were Varjack ordered the pizza)
The girls (not Ali) get kidnapped by A whilst they're being transferred and they wake up in the house and see mona lying on the floor in an Ali mask in the clothes she wore the night she went missing
A makes them do weird stuff like, plan prom and when they don't A plays a siren into the house because she's not happy, spencer and mona figure a way how to disconnect the power and they manage to escape from the house, but as they get out they realise there's a massive fence around the house
The ep ends (I think) with the girls face to face with a masked A