The world stands on the brink of nuclear armageddon. The Players are back, manipulating events for their entertainment, but this time the Doctor has no idea who they are.
The seventh Doctor has a brief cameo, reprising a scene from Timewyrm: Exodus.
Ace has a brief cameo at the beginning.
None. The box-like shell resides in the Doctor's cellar (pg 20) and is later transported elsewhere and then returned.
The preceding Earth arc books aren't necessary, but are nice if you're doing it in sequence.
Pg 3 "A small dark man and a tall dark-haired girl were drinking tea and eating buns at a Festival of Britain refreshment stall." We get a reprise of the beginning/end of Timewyrm: Exodus.
Pg 27 "The Doctor had a sudden vision of a many-armed glowing-eyed being in a misty cavern somewhere immensely far away." The Doctor briefly flashes back to his Venusian karate teacher, while engaging in the activity. The third Doctor used Venusian karate rather a lot.
Pg 40 "I know a man called Greene who might help." The Turing Test.
Pg 42 "Ask them to send over that Turing file that Greene compiled in the mid-forties." The Turing Test.
Pg 48 Reference to Turing and Greene (The Turing Test).
Pg 61 Reference to Greene (The Turing Test).
Pg 66 "But he seemed to have a natural gift for code-breaking - a gift sharpened by recent association with Britain's finest code-breaker, Alan Turing." The Turing Test.
Pg 67 "When he looked into the painted face of the Venerable Bede, he could imagine the man holding a fishing rod for some reason." The Doctor says the Venerable Bede adored fish in The Talons of Weng-Chiang.
Pg 75 Reference to Turing and Greene (The Turing Test).
Pgs 83-84"'Players...' He broke off suddenly, staring blankly into space. 'What is it?' 'Just for a moment I thought the word meant something to me.'" Players.
Pg 112 "Players... Somewhere deep within him, a faint spark of memory flickered... and died. Something about a woman. A beautiful and dangerous woman." Players.
Pg 118 "Liberty Hall, my dear chap. Liberty Hall!" Also said by the Brigadier in The Three Doctors.
"Sleep, after all, was for tortoises." The Talons of Weng-Chiang.
Pgs 125-126 "I like trains. I used to have a set myself." The Doctor's love of trains was first mentioned in Black Orchid and he mentions having a set in Love and War. We see the eighth Doctor playing with his in the short story Model Train Set from Short Trips.
Pg 136 "I seem to remember carrying some sort of screwdriver thingy with me, but I seem to have misplaced it." The sonic screwdriver, first seen in Fury From The Deep.
Pg 145 "There's no point in being grown up if you can't be childish occasionally." Robot.
Pg 214 "First the funny little clown, then that great handsome bull of a man..." The second and sixth Doctors, Players.
Pg 215 "Remember the expression on the Count's face when I shot him?" Players.
Pg 219 "I must know. I mustn't know!" This is the first, but by no means the last, time an all-powerful alien offered to restore the amnesiac eighth Doctor's memories and he recoils in horror at the idea. This appears to be motivated by his subconscious fear of confronting his destroying Gallifrey in The Ancestor Cell, but The Gallifrey Chronicles provides a much better resolution to this arc.
OLD FRIENDS AND OLD ENEMIES
The Countess, who returns from Players.
Kim Philby was briefly seen in The Turing Test, but plays a major role here. He'll also show up briefly in History 101.
NEW FRIENDS AND NEW ENEMIES
Guy Burgess, Donald MacLean, Harry S. Truman, Joseph Stalin, Jim Anderson, Kent Howard, Penny the waitress.