Some time after the crystal containing most of Iris' memories was shattered, Tom and Panda try to get use to life in the TARDIS again. Iris isn't making things easy for them, since all she does is drink, get sick and contemplate throwing her companions out again. When Tom goes out to buy her some medication, he meets a Mad Doctor named Zachariah Marwick who insists there's a lump inside Tom's head and he needs to cure it by going to a singles night once a week. Marwick promptly throws Tom down some steps and into said singles night, where he meets a very confused girl named Alice who indeed has a weird lump.
Meanwhile, Iris is suffering from previous companion Jenny's TARDIS travel sickness, Panda is suddenly in love with Iris and feels a heart beating inside his stuffing, and Tom finds straw coming out of his nose and ears. Also, no one can stop making lame sheep-related puns. Team TARDIS' memories cross over into their heads (Panda's a bit horrified when he can suddenly remember Iris'... escapades) and they hear weird voices whispering.
Marwick turns out to have spent the past 30-or-so years engineering the return of a Biblical demon, who was once defeated by Iris. He implanted bits and pieces of the demon into babies at a hospital, including Tom and Alice, so that the body parts could grow inside them. However, Iris' defeat of the demon wasn't absolute to begin with, and when it came back for her later that night, it nested itself in her mind... while she was watching The Wizard of Oz and counting sheep. Which explains why Iris is suddenly quite a cowardly lion, Tom has straw instead of a brain, Panda has a heart and Alice finds herself wearing ruby red slippers and has a miniature bus inside her instead of a demonic body part. Marwick tries to use Panda for a Virgin Sacrifice, but is stopped by the fact that Panda has some of Iris' memories now... and, although he's a virgin, Iris definitely isn't, so the sacrifice fails. The reason their memories leaked into each other's minds in the first place is the shattering of Iris' memory crystal, bits of which crept into her companions. And because Iris was counting sheep when the demon crept into her, it returns to earth as... well, as a sheep.
