“Illegal Alien” is/was-to-be a Cyberman story featuring the 7th Doctor and Ace.
“Illegal Alien” was under consideration as a three-part script for the 1990 season of Doctor Who. It was never produced as the original run of Doctor Who ceased production in 1989. A novelization of “Illegal Alien” (which is for the most part close to the original proposed TV story) was published as a (now out of print) novel by BBC Books, written by Mike Tucker and Robert Perry.
The story that became the novel was originally submitted as a television script to have been part of Season 28 of the original series of Doctor Who. It was initially submitted under a pseudonym as Mike Tucker had been known within the BBC as having predominately worked with special effects.
“Illegal Alien” (the unproduced TV story) was a Cyberman story set in the 1940s, originally submitted just prior to the production of “The Curse of Fenric Tucker and Robert Perry were asked to re-submit it for the following season as Fenric was also set in the 1940s.
Mike Tucker noted in Doctor Who Magazine about the writing of the novel (and its TV heritage) that –
“Illegal Alien was originally written in script form. When I (Mike Tucker) was writing the novel, the early part I was working from our original script to get all the dialogue for the novel, so that was quite nice for me – it was the novelising of an unmade story in effect. The opening two parts of Illegal Alien, when it finally hits the bookshelves, is damn near what was in the scripted version. When it does come out it may show some of the problems, in that it would have been a very expensive production, which could ultimately have been its downfall.”
(Doctor Who Magazine #255)