Warning: This story contains some minor plot details, which some readers may wish to avoid.

Socialism, skullduggery and scandal are the backbone of Downton Abbey, which returns to ITV this weekend for its fifth series.

The new season of ITV’s much-garlanded drama is set in 1924, as Britain’s first Labour government comes to power.

For the Crawley family, the changing political climate is unsettling. Socialists, says Earl Grantham, “are committed to the destruction of people like us and everything we stand for”.

But for some of the other characters, notably assistant cook Daisy Mason, the dissolution of the old social order presents opportunities she could never have imagined before the war.

Not everything changes, though. Maggie Smith’s Dowager Countess still gets all the best lines.

“Principles are like prayers,” she scolds one dinner guest. “Noble, of course, but awkward at a party.”

Ahead of the show’s return, we caught up with the cast to find out the secrets of the new series, both on and off screen.

THE PLOT IN A NUTSHELL (UPSTAIRS)

“Robert is scared,” says Hugh Bonneville of his character, the show’s soft-centred patriarch.

“I think there was a very real concern that, having had a Liberal alliance that was threatening the structure of society, a Labour government was definitely going to do that – so he’s really quite scared.

“Those concerns are characterised by the intrusion of Sarah Bunting, the schoolteacher who’s befriended Tom Branson. Under normal circumstances, he would be very happy to have a healthy debate with her about the political arena but the fact is she’s just a cow.

“So that provides a lot of entertainment – them being at loggerheads.

“There’s also some intrigue on the horizon with the arrival of Richard E Grant. Never trust an art historian who wants to look at your etchings.”

THE PLOT IN A NUTSHELL (DOWNSTAIRS)

The pivotal moment in the last series was the rape of lady’s maid Anna Bates, and the subsequent death of her attacker Mr Green – which may or may not have come at the hands of her husband, John Bates.

That story continues to have repercussions for the couple this year.

“It wouldn’t make any sense if what happened in series four was brushed over and everything was fine,” says Joanne Froggatt, who plays Anna. “It’s not going to be fine for a long, long time, if ever. There’s going to be a lot of issues for them along the way – physically, emotionally, all of those things you might expect.”

Meanwhile, devious under-butler Thomas Barrow (Robert James-Collier) gets a surprisingly sympathetic plotline, as he struggles to come to terms with his sexuality.

“He never apologised for being gay,” says the actor, “but in this series something happens and for the first time Thomas questions: ‘should I be gay?’

“He then goes on a path, which is quite an unhealthy path, to try and change who he is. It’s quite an emotional and quite a powerful storyline, I hope.

“He’s alone and he doesn’t want to be alone any more. The only way is to change who he is, and he can’t do that. It’s heartbreaking.”

LADY MARY TAKES OFF HER HAT

A quarter of an hour into the first episode, Lady Mary informs her parents, with all the gravitas and portent of Chamberlain declaring war, that she is “going upstairs to take off my hat”.

“I really liked that line,” laughs Michelle Dockery, “because it is the most uninteresting thing to go and take a hat off – but it’s part of her routine.

“She would go into town wearing a hat; her hair is styled so it suits the hat; and when she comes back and the hat is taken off, she’d have to have her hair restyled.

“So it’s a thing she has to plan as part of her day – taking off her hat. I’m glad it comes across with a bit of humour.”

THE FIRST RULE OF THE CHRISTMAS SPECIAL IS THAT NO-ONE TALKS ABOUT THE CHRISTMAS SPECIAL

“Can we tell you about the Christmas special?” says Jim Carter (Mr Carson), incredulously. “Not allowed to, they’d shoot us.”

He indicates an ITV publicist on his left: “This woman here, she’s a trained ninja. That looks like a water bottle, but it’s a gun and she’d shoot us dead if we talked about it.

“It’s on at Christmas, I can tell you that. There might be snow.”

He glances nervously at the publicist. “Tell me if I’m saying too much.”

ONLY ONE PERSON HAS TO WEAR A CORSET

“Mrs Hughes is the only one in a corset any more, I’ll have you know,” says actress Phyllis Logan, looking pained just at the thought of her restrictive costume.

“It’s hard to believe people did that every day. No wonder they were all getting attacks of the vapours every five minutes and fainting right, left and centre.”

“But it’s almost worth doing it for the relief you get at the end of 12 hours when you can actually take it off and go ‘phew.'”

WHATEVER YOU DO, DON’T EAT THE FOOD

At times it seems like all of Downton’s plot developments take place at dinner.

But don’t be fooled into thinking the cast are tucking in for real – almost all of the food is sprayed and treated with chemicals to keep it looking fresh for the camera.

“When it’s sprayed, it’s torture,” says Lily James, who plays the free-spirited Lady Rose. “It’s all wasted and I can’t bear it.”

Worse still – the same food gets re-used from one series to the next.

“Anybody who sees the lobster doesn’t want to be there,” says Lesley Nicol, who plays head cook Mrs Patmore. “It has been in and out of the freezer several times and it’s started smelling something terrible.

“It must have been a spectacular lobster for them to want to keep it. They just keep thinking: ‘we’ll give it one more go.'”

MAGGIE SMITH IS THE QUEEN OF BANANAGRAMS

“There’s a lot of downtime, so we amuse ourselves backstage with Bananagrams – which is like a Scrabble game,” says Lily James (Lady Rose).

“Maggie Smith loves it. They’ll be like ‘can we have you back on set’ and she’s just like [imperious wave of the hand] ‘not now.'”

“It keeps your brain active,” adds Michelle Dockery. “Occasionally a rude word comes out and we have a laugh.”

Everyone agrees that Dame Maggie is the undisputed champion, although Laura Carmichael (Lady Edith) hints that she bends the rules to her own advantage.

“Sam Bond (Lady Rosamund), who’s probably the best, knows all the two-letter words and Maggie gets very annoyed,” she laughs.

“And if you have to look a word up, she doesn’t really accept it.”

THE BITCH IS BACK

In 2013, the Crawleys’ golden Labrador was seldom on screen, leading some viewers to worry she’d been written out of the series.

“She was relatively quiet last year,” admits Hugh Bonneville, “but she’s been around a lot more this time, which is lovely. She’s just quite happy sitting there by the fire.”

In fact, he hopes the canine’s cameos could be beefed up even further.

“She could be like a St Bernard and go and rescue people,” he says, suggesting that Isis could even have tackled the fire that rages through the Abbey in episode one.

“She should have been in there, holding the hose in her teeth.

“We could even have an Isis spinoff. We’d call it ‘Move over, Lassie.'”

DON’T ASK MR CARSON FOR A SELFIE

The global success of the show – Downton is shown in almost 250 countries – has taken the cast by surprise, but not all of the attention is welcome.

“The worst was the British ambassador’s residence in Washington two years ago,” says Jim Carter.

“There was all the great and good of Washington there, all in their suits, and they behaved most inappropriately. They were tearing at us as if we were the Bay City Rollers.

“It got to be, “Yes, all right, I will take a selfie. Back off!'”

Has he perfected his selfie pose, though?

“Absolutely not,” he reprimands.

“If people ask me in the street, I tend to politely decline and offer an autograph because (a) nobody knows how to use their camera and (b) it attracts too much attention.

“I don’t approve of selfies,” he adds.

JOINING THE CAST IS A TERRIFYING EXPERIENCE

The cast are horrified at the suggestion new actors might find it daunting in Downton.

“I think it’s actually very nice for them because we’re all so bored of each other,” laughs Michelle Dockery.

“We’re like sharks circling in a big pool,” agrees Hugh Bonneville. “When a bit of fresh meat is thrown in, we all dive on it. We ripped Richard E Grant and Anna Chancellor to shreds for several weeks.”

But Daisy Lewis, who joined the cast last year as outspoken schoolteacher Sarah Bunting, has a very different take on her first day on set.

“I’m still sort of getting over it,” she says. “I’m still very much at the beginning of my career so it was pretty nerve-wracking. I had to dig pretty deep to have the confidence to walk into that room.

“My character is very isolated as well. She’s one very small girl up against quite a large system, and I think that that’s how I felt as an actor. So it was actually helpful, in a way, because I could use it to feel how she would have felt – how she would have had to brace herself to walk in there.”

HINTS ARE MADE ABOUT THE US SPIN-OFF

A US spin-off of Downton Abbey, tentatively called The Gilded Age, has been on the cards since 2012, and a throwaway line in series five refers to “Tom’s American plans”.

Is this a hint that the Allen Leech’s popular character, a former chauffeur and widowed husband of Lady Sybil, could front such a show?

“Series five for [Tom] Branson is definitely a series where he has to discover exactly what he wants out of life, and whether his life really is at Downton, or whether he could be more comfortable in another world,” says Leech.

“But the only American spin-off that’s going to happen is when Barrow and Branson go out, and Branson sets up an Irish Bar and Barrow sets up the first Edwardian gay bar across the road… and hilarity ensues.

“Wait until you see the first script. The pilot is unbelievable.”

The new series of Downton Abbey starts on ITV on 21 September at 2100 BST