The Gassy Immigrants was the third episode of Series 1. It stars Ecclescake and Billie Piping and was written by noted UKIP member Mark "RaHoWa" Gatiss.
Nine and Rose end up in Cardiff in 1869 where they meet Charles Dickens, Sneed (formerly Chuck), and some rather nasty, gassy immigrants. Fortunately a retarded ancestor of Gwen from Torchwood blows herself up and takes the immigrants with her.
The Cardiff Rift plot thread begins here, running through the rest of the RTD era like a fault line, and with much the same effect.
Is this technically a christmas special?
Note: Only Autists think this episode is racist, so far as we can tell.