4T. Doctor Who - The Invisible Enemy

Season Fifteen

  • Script Editor: Robert Holmes
  • Writers: Bob Baker and Dave Martin
  • Produced by: Graham Williams
  • Director: Derrick Goodwin
  • Designer: Barry Newbery
  • Incidental Music: Dudley Simpson

4T. Doctor Who - The Invisible Enemy

A spacecraft en route to the Titan base is infected by a virus, and the same virus - the Swarm - also attacks the TARDIS. The Doctor is infected by the Nucleus of the Swarm. Landing at the Titan Base the Doctor and Leela are threatened by the crew. The Doctor and Leela manage to get away to a hospital for the Doctor - the Bi-Al Foundation.

Professor Marius is able to clone the Doctor and Leela, minaturise them, and inject them into the real Doctor. The hope is that they will be able to find the infection and destroy it.

The Swarm nucleus manages to get out and is enlarged to human size. It arranges for transport back to the Titan base where breeding tanks have been prepared for it.

The now cured Doctor, along with the help of Professor Marius's mobile computer K-9, manages to set a trap which will blow up the breeding tanks. As a parting gift Professor Marius leaves K-9 in the care of the Doctor.

Regular Cast

  • Leela: Louise Jameson
  • Voice of K-9: John Leeson


  • Supervisor Lowe: Michael Sheard
  • Captain Safran: Brian Grellis
  • Meeker: Edmund Pegge (1)
  • Silvey: Jay Neill (1)
  • Crewman: Anthony Rowlands (1)
  • Professor Marius: Frederick Jaeger (2-4)
  • Opthalmologist: Jim McManus (2-4)
  • Doctor Parsons: Roy Herrick (2-4)
  • Marius's Nurse: Elizabeth Norman (2-3)
  • Doctor Cruickshank: Roderick Smith (2-3)
  • Doctor Hedges: Kenneth Waller (2)
  • Reception Nurse: Nell Curran (2)
  • Nucleus: John Scott Martin (3-4)
  • Nucleus Voice: John Leeson (3-4)
  • Medic: Pat Gorman (3-4)
  • Crewmen: Harry Fielder (1), Stuart Myers (1)
  • Computer Voice: Anthony Rowlands (1)
  • Bi-Al Members: Leslie Bates, Alan Clements, Margot Gordon, Derek Hunt, Kenneth Sedd, Cy Town

Showings on the BBC

First showing

Channel Episode Title Date Viewers Rating
BBC 1 458 Part One Saturday, October 1, 1977 6:20 PM - 6:45 PM 8.6M
BBC 1 459 Part Two Saturday, October 8, 1977 6:05 PM - 6:30 PM 7.3M
BBC 1 460 Part Thre Saturday, October 15, 1977 6:10 PM - 6:35 PM 7.5M
BBC 1 461 Part Four Saturday, October 22, 1977 6:10 PM - 6:35 PM 8.3M

Repeat showings


Outside Locations

No location details are available


  • Television Centre Studio 6



No blooper details are available


  • K-9 does not appear in part one.

Working titles

  • The Enemy Within
  • The Invisible Invader