The couple clash bitterly at Home Farm in the coming days, and when Declan heads off on a quad bike for some time alone, it sparks a terrible chain of events as Katie (Sammy Winward) climbs on another one to follow him.
While the pair are both out on the bikes, disaster strikes for Katie when she falls - quad and all - down a disused mine. Declan tries his best to rescue her, but when his efforts make the situation worse, things look incredibly bleak…
Here, Jason Merrells - who plays Declan - answers some quickfire questions about the storyline.
Is Declan's marriage to Katie already in trouble?
"Well, there's a worm that has got into Declan's head now! It's about Katie lying to him over the whole Robbie situation and how she set him up to get rid of him. Even though what Robbie did was way beyond the pale, Declan has become a little bit fixated on the fact that Katie was dishonest.
"In a silly way, Declan had held Katie up as an icon of virtue, but now things have gone wrong and he's worrying that he can't trust her. Personally, I think it's a bit of a storm in a teacup, but like men can do sometimes, Declan runs away with it. He gets himself into a bigger argument than he intended, and it spirals out of control..."
Is that how the mineshaft storyline comes about?
"Yes, Declan has been having another argument with Katie when he goes off on a quad bike. Basically, he storms out and goes off! Katie follows him on another bike and it all stems from there, building up to the moment where she falls into the mineshaft."
How was the storyline filmed?
"There were lots of different set-ups, so the team had to be quite clever about it. They dug a hole out on-location for the exterior scenes, and then we had the interior of the mine in the studio, which looked amazing. So it was all filmed in different stages and put together.
"Some bits weren't exactly as they seemed - we had to fake the depth of the mine at certain points, and pretend that the ground was uneven when really it wasn't, because to build an entire mineshaft would have been out of the question! We did our best and I think it will look really good on screen."
Were you blessed with good weather for the outdoor scenes?
"It was actually really tough! It reminded me of the John Barton stunt that we did in winter, except that it wasn't minus five temperatures for this one! The conditions were tough, but I think that brings us together as a team."
How does Declan react after the accident?
"Declan is absolutely in bits - he's a complete mess. He realises that he was being silly by arguing so much with Katie, and now he could have lost her completely. He goes into a meltdown about it."
Katie shouts up to Declan that he shouldn't leave her alone, but that's exactly what he does as he goes to get help. Is that a difficult choice to make?
"I think that's a reasonably good decision that he makes. He feels bad about leaving Katie, but he has to do something because he has no phone.
"You'll see that Declan runs to the nearest farm, which is Moira's farm, and he gets a tractor to help rescue Katie himself. What he's doing is pretty logical and it's what most rescue teams would do - get a rope down there and tow her out.
"Unfortunately, Declan pulls the tractor too near the hole and there's a further collapse, which causes Katie to fall a bit deeper. So he feels even more guilty after that, because he's made things ten times worse…"
What happens next?
"You'll see that things get very dark for Declan. Katie falls so far down that she disappears and they can't hear her anymore. In the end, there comes a point where the official rescue teams begin to slow down - they're starting to phase out the attempts to save Katie.
"Declan has very intense emotions - he just thinks that he's killed Katie. He spirals downwards into self-hatred as a result…"
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Emmerdale airs Katie's mineshaft accident on Thursday, November 22 at 7pm on ITV1.