The TARDIS brings the Doctor and Martha to Balmoral in 1902. Here they meet Captain Harry Carruthers - friend of the new king, Edward VII. Together they head for the castle to see the king - only to find that Balmoral Castle is gone, leaving just a hole in the ground.
Pg 6 On a scottish hillside, 1902.
Pg 26 Picadilly Circus.
Pg 39 On top of a sand dune, in the Arabian desert (pg 35).
Pg 83 Just outside Balmoral Castle (now returned to its rightful location).
Pg 99 In the laboratory of the Cosmic Peacemakers, Black Horse Lane (but see Continuity Cock-ups).
It helps a tiny bit to be familiar with Smith and Jones.
Pg 7 "Queen Victoria loved this bit of countryside/ Called it her Highland paradise." Tooth and Claw.
Pg 10 "'My heart's in the Highlands, my heart is not here,' sang the Doctor in a surprisingly good Scots accent." Reference to David Tennant's natural Scottish accent, which we hear in Tooth and Claw.
Pg 15 "The hospital where Martha had been a medical student had been whisked away to the moon by the Judoon." Smith and Jones.
Pg 22 About Arthur Conan Doyle: "I met him once." Evolution.
Pg 60 "We are the Peacemakers." It's not actually a reference to Peacemaker, the immediately prior book, but the aliens there were also named Peacemakers (see page 146 of that book).
Pg 65 "Queen Victoria thought I was a bad influence..." Tooth and Claw.
Pg 68 "And the Daleks brought out a new Master Plan every week." The Daleks' Master Plan et al.
Pg 98 "I shall recruit other soldiers - warriors from Schlangi, or Ogrons perhaps." The Ogrons first appeared in Day of the Daleks.
OLD FRIENDS AND OLD ENEMIES
The Judoon, who first appeared in Smith and Jones.
Sir Arthur Conan Doyle, who previously appeared in Evolution.
NEW FRIENDS AND NEW ENEMIES
King Edward, Harry Carruthers.
Pg 60 The Peacemakers, lizard-like aliens who can camouflage themselves as humans.