"Christmas Eve in the year 3060, and the planet Puxatornee is home to a prosperous human colony.
"A space craft has arrived in orbit carrying the Slithergees, a race of obsequious alien slugs. Their home world has been destroyed and they are humbly requesting permission to settle on the first moon.
"And if they don't get permission, then they are humbly threatening to declare all-out war.
"The future hangs in the balance. The decision rests with Bailey, the colony's president - but she has other things on her mind...
"Christmas Eve in the year 3090, and the planet Puxatornee has changed beyond all recognition.
"The Doctor and Mel arrive, on a completely unrelated mission to defeat a race of terrible monsters, and soon discover that something rather confusing has been happening to history..."
The oddly titled "Flip-Flop", by Jonathan Morris, is another experimental outing by Big Finish that turns out to be a great success. By an incredible feat of narrative complexity, Morris has created a four-part story split over two CDs that can be listened to in either order.
The principal conceit of "Flip-Flop" revolves around two parallel versions of history, originating from a single branch point thirty years previously, in both of which the Doctor and Mel arrive and end up becoming embroiled in a plot to change the history of the planet. Without spoiling too much of the plot, the result is a paradox in which the history of Puxatornee changes endlessly from one version back to the other. Neither is correct, and neither is incorrect, and it is entirely down to the listener's choice as to which CD to listen to first (the "black" CD or the "white" CD) to determine which version of history the listener is eventually left with.
Aside from the ingenious plotting that allows the events of both CDs to be reflected in the other, "Flip-Flop" isn't perfect. The human supporting characters are largely one-dimensional and serve only to implement the over-arching plot device. The Slithergees, however, are a great creation, being both sickeningly obsequious but utterly in control of the humans at the same time, and their lugubrious, gurgling voices only add to the effect. "Flip-Flop" is also somewhat depressing: either path through the history of the planet gives rise to utter misery for its inhabitants. However, with a clever conceit at its core, great performances from Sylvester McCoy and Bonnie Langford and a well-crafted downbeat score, "Flip-Flop" is well worth a listen.