The Thals and the time travellers attack the Daleks from all sides as the Daleks start the countdown to release deadly doses of radiation into the atmosphere.

With the commerce of the world grinding to a halt, the Doctor, Jamie and Zoe agree to investigate. They set off for the T-Mat Relay Station on the Moon in Professor Eldred's old fashioned rocket.

When the Doctor restores the power to the Ark, the humans begin to revive. But the station has been invaded by the giant insectoid Wirrn.

Born in Liverpool in February 1946, Elisabeth Sladen took an interest in acting from an early age; she joined the Liverpool Playhouse repertory company as assistant stage manager after drama school, where she would meet her future husband Brian Miller. After a stint touring the country in repertory, she settled in Manchester, during which she had her breakthrough television role in 1970, appearing as Anita Reynolds in Coronation Street. More small roles were to follow in popular shows like Doomwatch, Z Cars and Some Mothers Do 'Ave 'Em.

Then in 1973 came what would be her defining role as she was cast as the replacement companion for third Doctor Jon Pertwee. Following in the footsteps of popular companion Jo Grant played by Katy Manning, Sladen was to hit the ground running as investigative journalist Sarah Jane Smith in her debut story The Time Warrior, and over the next three years would see her popularity soar as she became part of what is arguably known as the "golden years" of Doctor Who alongside fourth Doctor Tom Baker, an era including the story frequently appearing top of fan polls, Genesis of the Daleks.Despite choosing to leave the programme in 1976, with an emotional departure scene at the conclusion of the serial The Hand of Fear, 

Sladen later stated in interviews that although she had left Sarah Jane, "Sarah Jane never left me". Although she declined producer John Nathan-Turner's offer to come back as support for the Fourth Doctor's regeneration (being present at his own arrival at the end of this week's DVD release Planet of the Spiders), she did returned to the role for the 1981 pilot: having been described by the Doctor as his best friend, Sarah was form her second long-lasting partnership with his "second-best friend" in the titular K9 and Company.Sladen was re-united with "her" Doctor, Jon Pertwee, in the 1983 20th anniversary celebration The Five Doctors, and again during the 1990s for the two radio stories The Paradise of Death and The Ghosts of N-Space, written and produced by the person who originally cast her, Barry Letts - the two would also be joined by another popular Doctor Who legend in the form of Nicholas Courtney as the Brigadier.

Peter Sallis, OBE  is an English actor and entertainer, well known for his work on British television. 

Sallis is best known for his role as the main character Norman Clegg in the long-running British TV comedy Last of the Summer Wine, set in a Yorkshire town. He was the longest serving cast member, appearing in all 295 episodes, and by the end of the show's run was the only one surviving from the programme's first episode in 1973. He also appeared in all 13 of the episodes of the prequel series First of the Summer Wine as Norman Clegg's father. He is also famous for providing the voice of Wallace in the Wallace and Gromit films.

He played Elric Penley in the 1967 Doctor Who serial The Ice Warriors. He was later due to play Striker in Enlightenment but industrial action caused filming times to change and his schedule could no longer fit the role in

Sallis was born on 1 February 1921 in Twickenham, Middlesex, England, the only child of Dorothy Amea (n�e Barnard) and Harry Sallis. After attending Minchenden Grammar School in North London, Sallis went to work in a bank. After the outbreak of World War II he joined the RAF. He failed to get into aircrew because he had a serum albumin disorder and he was told he might black out at high altitudes. He became a wireless mechanic instead and went on to teach radio procedures at RAF Cranwell.

Sallis started as an amateur actor during his four years with the RAF when one of his students offered him the lead in an amateur production. His success in the role caused him to resolve to become an actor after the war, and so he trained at the Royal Academy of Dramatic Arts, making his first professional appearance on the London stage in 1946.

Sallis married Elaine Usher at St John's Wood Church in London on 9 February 1957. Their son, Timothy Crispian Sallis was born in 1959.

Joan Young played Catherine de Medici in the 1966Doctor Who story The Massacre of St Bartholomew's Eve.

Other credits include All Creatures Great and Small, Salvage 1, The Chiffy Kids, Shades of Greene, Sykes, Billy Liar, The Stanley Baxter Show, Z Cars, The Wednesday Play, No Hiding Place, Dixon of Dock Green, Dr. Kildare, Doctor Knock, The Citadel, The Inn of the Sixth Happiness, Maverick, Anne of Green Gables

Robin David Sachs was an English actor.

Sachs was born in London, the son of actors Leonard Sachs and Eleanor Summerfield. He was married twice: to Welsh actress Siân Phillips (1979–1991), and to Casey Defranco.

Among his roles, he played the evil magician Ethan Rayne in Buffy, The Vampire Slayer as well as General Valen in Star Trek: Voyager and Coplann in Babylon 5: In The Beginning. He also made three different guest appearances in Babylon 5 and was the warmongering General Roth'h'ar Sarris in the sci-fi comedy parody film Galaxy Quest.

Reginald Jessup appeared in two Doctor Who stories: as the Servant in the 1966 story The Massacre of St Bartholomew's Eve and Savar in the 1978 story The Invasion of Time.

Also appeared in The Bill, Shine on Harvey Moon, The Tragedy of Coriolanus, Play for Today, Escape, Hamlet, Prince of Denmark, Julius Caesar, Rebecca, In the Forest, Target, 1990, The Signalman, Softly Softly: Task Force, Z Cars, The Befrienders, Dixon of Dock Green, Hardy Heating Company Ltd, Department S, Softly Softly, The Champions, The Avengers, Gideon's Way, The Baron, The Spies, Theatre 625, Reluctant Bandit, The Wednesday Play, The Saint, Curtain of Fear, Maigret, The Sword in the Web, BBC Sunday-Night Play, Silent Evidence, Sir Francis Drake, Boyd Q.C., Drama 61-67, Danger Man, Fact and Fiction, No Wreath for the General, Here Lies Miss Sabry, Armchair Mystery Theatre, Blood Wedding, This Desirable Residence, ITV Play of the Week, Hour of Mystery, Vanity Fair, The Count of Monte Cristo, Quatermass II, Puck of Pook's Hill