Note: One of 4 Doctor Who books published as part of the "Bug Club" schools literacy programme. This one is a 48-page "photo novelisation" written for Year 5 pupils (i.e. 9-10 years old) and obviously based on the 2010 episode The Lodger, while the other books are based on The Eleventh Hour, The Time of Angels and Victory of the Daleks. 4 similar Sarah Jane Adventures books (though aimed at an even younger audience) were released in 2010: Judoon Afternoon, The Haunted House, Painting Peril and Blathereen Dream.
There's a house in an ordinary street in an ordinary town with a staircase that people go up, but never come down... To solve the mystery of the man upstairs, the Doctor must pass himself off as a normal human being, and share the downstairs flat with Craig Owens.