assisted them. Prof. Marius offers K9 to the Doctor, as he is due to return to Earth, and the Doctor and Leela leave with their new companion in the TARDIS. Production Working titles for this story included The Enemy Within, The Invader Within and The Invisible Invader. It was not decided until late in the production that K9 was to be a new companion. The decision to use it in multiple serials was made partly to offset the expense that had gone into making the prop. The Invisible Enemy was filmed and recorded in April 1977. In one scene there is an obvious crack in a wall before it is fired at by K9; the crack was originally concealed, but the scene was reshot with little time left to repair the join. Cast notes Michael Sheard (Lowe) makes his fourth of six appearances in Doctor Who, having made previous appearances in The Ark (1966), The Mind of Evil (1971) and Pyramids of Mars (1975). Brian Grellis previously played Sheprah in Revenge of the Cybermen (1975) and would later appear as the Megaphone Man in Snakedance (1983). Frederick Jaeger (Marius) also played Jano in The Savages (1966) and Professor Sorenson in Planet of Evil (1975). Broadcast and reception The story was repeated on BBC1 on consecutive Thursdays from 13 July to 3 August 1978, achieving ratings of 4.9, 5.5, 5.1 and 6.8 million viewers, respectively. Reviewing the serial for The Times newspaper on the Monday following the second episode's transmission, critic Stanley Reynolds gave the story a generally negative reception. He also pointed out that in ITV regions where the series was competing with Man from Atlantis in the Saturday early-evening slot, it was now losing the ratings war: More recent reviews have also not been positive. Paul Cornell, Martin Day and Keith Topping wrote of the serial in The Discontinuity Guide (1995), "An ambitious project which has the look of a grand folly due to budget constraints and the tongue-in-cheek script... K9 makes a quite impressive debut, though, as with many aspects of The Invisible Enemy, the ideas are better than the realisation." In The Television Companion (1998), David J. Howe and Stephen James Walker called it one of the "weakest" Fourth Doctor stories, mostly consisting of "clichéd and undemanding action-adventure material". They also noted the inconsistent visual effects. In 2010, Mark Braxton of Radio Times awarded it two stars