Scientists expect there to be consequences to their work – but none of her colleagues expect Dr Marshall to react quite so badly…

That’s Doctor Leela Marshall, whose experiment in March 1978 is going wrong: she refuses to accept reality, and maintains that she is Leela of the Sevateem, a non-native of Earth, who has been travelling with someone called the Doctor… who’s the spitting image of her colleague Dr Stewart. That’s the conceit at the heart of David Llewellyn’s excellent new adventure for the Fourth Doctor and Leela, and it’s one that plays out in a number of unexpected ways. Of course, we as listeners (and Leela herself) are sure that this new reality is wrong, but could there be a shadow of doubt, particularly with everyone (including a surprising reappearing guest star) telling her that she’s wrong?

Star Trek: Deep Space 9 ran with a similar idea with its Benny Russell subplot (and at one stage, it was even contemplated that the show would end with Benny finishing his story of the aliens and the wormhole), but Llewellyn only has an hour or so to play with, and maintains the tension throughout. I was half expecting an injoke reference to Department 7 (the organisation that Louise Jameson’s character works for in The Omega Factor) as the Crowmarsh Institute bears certain resemblances, and there’s a great role for Julian Wadham as a psychiatrist trying to help Leela. Cathy Tyson and Damian Lynch are both heart-rending, but the story centres around Louise Jameson, who gives one of her best performances as Leela.

Nick Briggs’ direction and Jamie Robertson’s sound design are key to ensuring that we’re never pulled out of the created worlds in the wrong way – and there’s a good use of silence, the “dead air” that is often rightly feared in audio work.

Verdict: One of the best Fourth Doctor tales in recent times. 10/10

Paul Simpson