Roots: Pink Gin is (as referred in the play) a play on the name Red Rum? The mysterious disappearances of Lord Lucan and Shergar. Spindleton quotes from Blake's Jerusalem ("green and pleasant land") Richard II Act 2(i) ("This blessed plot... this England")
Technobabble: A temporal pulse may easily close a wormhole.
Dialogue Triumphs: "I've been trapped to this table before - I never forget a table!"
Continuity: The Kraals here are represented by the Second Kraal Army (the first having been defeated by the Doctor and UNIT in Devesham) Among their arsenal is a matter-dissolving ('M.D') bomb. The Doctor;s image is stored in their databanks.
The Doctor tells Leela that the Kraals have endured twelve centuries of exposure to radiation. O radiation is the predominant form of radiation on Oseidon and is harmful to life. The Doctor says there are other Kraals in subterranean bunkers on Oseidon. The First Kraal army under Styggron was defeated by the Doctor and UNIT (see: Links), while Grinmal leads the Second Kraal Army.
Z radiation is created by the slow decay of isotopes Alpha, beta and Omega in a flux-limited temporal field. Z radiation cannot be found on Earth naturally.
The unstable hybrid Z-O-radiation is formed when fields of Z-radiation and O-radiation converge and react. Prink's Theorem of 4795 postulates that Z-O radiation is able to emit the same level of energy as a billion neutron stars.
The Kraals made a further imitation Devesham on Oseidon. The real Devesham had a motorway built next to it and is (by Spindleton's description) something of a tourist trap.
While the Brigadier is in Canada with a team following up a case of some pylons coming to live in the countryside, and Colonel Faraday is in Geneva, Captain Clarke is the acting commander of the British division of UNIT. He is Greyhound Leader, UNIT HQ is Trap One
Colonel Hugh Spindleton was put onto the Master by his friend 'Lucky' Lucan, who believed the Time Lord would be just the man to 'put things right' with England. It may be inferred that the Master was in some way responsible for Luca's disappearance. Spindleton's horse Pink Gin is in fact Shergar, a race horse which stolen from Ireland in 1983 by the Master as a down payment for the Colonel's services.
The (unseen and unheard) Brigadier is still working for UNIT, and still remembers the Doctor.
The Master tells Leela that the last time that he met the Doctor was on Gallifrey (see: Links) His TARDIS continues to be in the form of a grandfather clock, and he has a staser (presumably of Gallifreyan origin) which is ineffective against Kraal androids.
Leela has a spare knife in her room on board the TARDIS. She loses her initial knife in this story, confiscated by Spindleton.
Time Lords possess a psychic empathy field which enables them to hypnotise others, and telepathic intuition, by which they may identify another Time Lord in their proximity.
A Z-battery is roughly the size of a carriage clock and highly volatile - they can play havoc with a TARDIS' navigational system.
Q.v.: 'UNIT Call Signs', 'Dating the UNIT Stories'
Untelevised Adventures: The Doctor and Leela drop Shergar off in the New Forest, a couple of centuries before the present day.
While on Earth, the Third Doctor attempted (and failed) to use a Z-battery to jump start his TARDIS. He left the Z-battery behind him in his lab, a fact gleaned by the Master.
Location: Earth,1979 again, including UNIT HQ in London and Oseidon.
The Bottom Line: 'I refuse to be defeated by myself!'
Actually better than you'd think. But not a great cliffhanger. Barne's tale of android duplicates, alien invasions by some of the least-memorable of Seventies baddies and (let's be frank) an anachronistic Master holds together, just. There's heroism here, and it's Leela's, and while the Master eventually really truly one hundred per cent does feature, his threat to the planet is oddly pedestrian (it's all about a battery? Really?) Could have been better.