'The Invasion of Time' (6 Parts) Story Code '4Z' by David Agnew++

After rendezvousing in space with a mysterious alien ship, the Doctor takes the TARDIS back to Gallifrey. Arriving at the Capitol, he demands to see Cardinal Borusa, and invokes his right to the presidency. As Leela becomes concerned at his increasingly erratic behaviour, the Doctor is crowned with a device that gives him access to the Matrix, the sum of all Time Lord knowledge. He then arranges for K-9 to drop the protective transduction barriers around the planet, and his alien allies, the telepathic Vardans, appear in the Capitol. These invaders can travel along any wavelength, including thought. While Leela befriends Rodan, a Time Lady, and Andred, a member of the Chancellery Guard, the Doctor reveals that after he discovered the Vardans' plan to invade Gallifrey, he began the process of setting a trap for them; his erratic behaviour is a way to confound their telepathy, keeping them unaware of his deception, until the time they materialise fully, which will enable him to locate their planet of origin, and trap it in a time loop. Fearing that Leela will unwittingly reveal his plans, he banishes her from the Capitol, sending her into the wastelands of the planet. Here she meets a group of outcasts, the Shobogans, and together they storm the Capitol. The Vardans fully materialise, only to be banished by the Doctor. However, before celebrations can begin, Gallifrey is invaded by the Sontarans, who have secretly been using the Vardans to pave the way for their invasion. The Time Lords are betrayed by the cowardly Castellan Kelner, who assists the Sontarans in securing the Capitol. The Doctor and his friends flee to the safety of the TARDIS, where the Doctor hypnotises Rodan, and sets her and K-9 to work on building the forbidden De-Mat gun, using knowledge gleaned from the Matrix and a sacred artefact, the Key of Rassilon. The Doctor operates the gun, killing the Sontarans, but his memory of recent events is wiped in the action. Leela decides to remain on Gallifrey to marry Andred, and K-9 elects to stay with her. Back in the TARDIS, the Doctor pulls out a cardboard box bearing the legend K-9 MkII...

Tom Baker (Doctor Who), Louise Jameson (Leela), John Leeson (Voice of K9); Stan McGowan, Tom Kelly (Vardans); Christopher Tranchell (Andred), Milton Johns (Kelner), John Arnatt (Borusa), Denis Edwards (Lord Gomer), Reginald Jessup (Lord Savar), Charles Morgan (Gold Usher), Christopher Christou (Guard), Hilary Ryan (Rodan), Ray Callaghan (Ablif), Michael Mundell (Jasko), Michael Harley (Bodyguard), Max Faulkner (Nesbin), Gai Smith (Presta) [3-6], Eric Danot (Castellan Guard), Derek Deadman (Stor), Stuart Fell (Sontaran)

Produced by Graham Williams
Directed by George Gerald Blake

TX:
Part 1 - 4th February, 1978 @ 6.25pm - 6.50pm
Part 2 - 11th February, 1978 @ 6.25pm - 6.50pm
Part 3 - 18th February, 1978 @ 6.25pm - 6.50pm
Part 4 - 25th February, 1978 @ 6.25pm - 6.50pm
Part 5 - 4th March, 1978 @ 6.25pm - 6.50pm
Part 6 - 11th March, 1978 @ 6.25pm - 6.50pm

Notes:
*Featuring the Fourth Doctor, Leela and K-9, and introducing K-9 MKII

++For this story, 'David Agnew' was a pseudonym for Graham Williams, Anthony Read and David Weir