The Mattress is back with a crystal from Ancient Atlantis (how he stole it from under Professor Zarroff's nose we'll never know), and he's using it on a machine called TOMTIT in order to bring a powerful and dangerous entity that lives outside of time known as Kronos to do his bidding. Also, a bunch of other shit including a TARDIS inside a TARDIS inside a TARDIS before Logopolis and that other thing did it, the Brig being frozen in time, and the Master having a kinky sex slave in an ancient civilisation. Yeah, it's one big clusterfuck.