Fans of Outlander are really keen to discover exactly how American author Diana Gabaldon will end Jamie Fraser (played by Sam Heughan) and Claire Fraser’s (Caitriona Balfe) story.

There has been plenty of speculation over the moment a man appearing to be Jamie stood outside Claire’s inn window in the very first book.

But isn’t it odd Frank (Tobias Menzies) could see the 18th Century Highlander when only moments later he evaporated into thin air, and can’t time travel like Claire can?

Many are convinced Jamie was actually a ghost, but what if Frank was secretly Black Jack Randall, the MI5 officer’s doppelgänger and ancestor?

It’s always been a puzzle over exactly why Frank and Black Jack look exactly the same (and are played by the same actor on the STARZ TV series), so it wouldn’t actually be too much of a stretch to learn Gabaldon had Black Jack playing the greatest role of his life.

What if Frank existed, but looked very different as a child, and was killed by the despicable Black Jack when he travelled through the Standing Stones to the future?

What’s more, perhaps the whole reason why Jamie was able to travel through time and space as a ghost or otherwise, and appear to Claire in Inverness, was actually because he sensed his wife was in danger, and had learned Black Jack had gone after her in the future.

How likely is it that Frank and Black Jack would look exactly the same, despite their shared ancestry? Surely there would be some differences.

There are flaws to the theory – for one thing, Gabaldon has said on a number of occasions the reason for Jamie’s appearance in the future will be explained in the last paragraph of the tenth book, and it would be a colossal disappointment if fans learn of such a big twist in the very last sentence, with no explanation to come.

There is also no way the completely despicable Black Jack could have hidden his true nature for that many years, or raised a daughter in Brianna (Sophie Skelton), but there were moments the character showed his dark side – like when he discovered Claire and Jamie were set to die in a house fire in the 1700s and chose to hide the truth from Brianna and his wife.

And Outlander is a story about time travel, after all.

One Reddit fan gave the final clue, and it all comes down to their views about Gabaldon’s novella about Roger MacKenzie (Richard Rankin), where Frank features heavily.

LullabyInBirdland devoured Gabaldon’s offshoot, but was left feeling incredibly emotional by the heart wrenching story of Roger’s history.

And what they may not have realised, is they pointed out a major plot point which could hint there’s a ginormous twist to come.

They wrote: “1. I cried in the end. Really. I’m quite heartbroken. 2. Did Frank realise who Roger was when he met him at Reverend Wakefield’s house?

“3. I really needed just one more paragraph, in the very end, along the lines of Jerry seeing Roger’s moss-green eyes as he held him, realising these are the same eyes he just saw in the stone circle, his last words being ‘I love you too, son’. Or something like that.

“4. How come Jerry was only two weeks in the past, but came back a year later?” the fan went on. “5. Why did Frank write about Jerry to Claire? It’s a great way to advance the plot of the story, but isn’t an MI6 officer supposed to keep this kind of information secret?”

But when the Reddit user got to their sixth point, the startling connotations could suggest the evil Black Jack has actually been masquerading as his future relative.

“6. Did we encounter Frank’s army rank anywhere before this story? I was really amused to see he was a Captain Randall. How very Black Jack of him,” the fan posted.

The theory is wild for sure, but stranger things have happened in the Outlander world.

Outlander book nine Go Tell The Bees That I Am Gone is expected to be released in 2020.

Outlander season five returns February 12 on STARZ.