After Kate moved out (back to the US? never really explained) Lee is faced with losing the flat and his rather inadequate cleaner Barbara. Lee rents the spare room to Tim's younger sister, Lucy.
Of course this doesn't work out.
Tim obviously becomes suspicious of Lee's influence on sister, in the same way as he as with Kate (his ex-girlfriend, in season 1.
Series 2 sees Lee finds Lee in the art world, the underworld, and showing some tenacity in trying to obtain football tickets.
Series 1 was very different.
Things used to be great between Lee and Kate, two flatmates who counted Tim as thier best friend and boyfriend respectively. But ever since Tim and Kate broke up, things are more complicated. Tim can't get over the break-up, and how is he supposed to handle the deepening friendship between Lee and Kate?