If you've not seen the previous series of Last Tango in Halifax or you've got a hazy memory, here's a guide to what's happened so far, who plays who and where you've seen the actors before.
Celia (Anne Reid)
The Last Tango story begins with Celia, a 70-something widow, who gets in touch with childhood sweetheart Alan after she spots him on Facebook. They meet up and get on so well that it's like they've never been apart. By episode two they're engaged, much to the shock and confusion of their children. The duo are based on writer Sally Wainwright's own mother's experience of a meeting her teenage boyfriend 60 years later on Friends Reunited. Wait, so there is hope I could end up marrying Sam, who I fancied in year nine? Interrrrresting....
Anyway, Celia's daughter Caroline is going out with a woman named Kate in series two and unlike Alan, Celia doesn't take the same-sex thing well at all. In fact, she refuses to accept it and there are torrid, throw-plates-at-the-wall, rows until Celia realises she's making her daughter miserable with her homophobia. As the Christmas special begins, Celia is happily married to Alan and is caught up in the daily chaos of her family. (Some) lessons have been learnt although her sharp tongue hasn't lost its edge.
Played by: Anne Reid, who you'll know as Valerie in Coronation Street and Jean in dinnerladies.
Alan (Derek Jacobi)
Alan knows all about life's peaks and troughs. He gets engaged to Celia and then at the end of series one he has a heart attack. It's all ok though because he recovers and marries Celia as planned. Alan's daughter Gillian has a complex time in the show and he's very involved - but more about that later. The crucial thing to know about Alan is that he discovers he has a son called Gary from an affair he had in the 1980s while married to his first wife, his daughter Gillian's mum. Alan doesn't tell Celia about Gary because he's worried she'll judge him for having had an affair - and especially because Celia's own husband was adulterous and she suffered badly. When he does tell her, she's devastated and genuinely questions whether he's the man she thought he was. By the time series four begins, they're reunited and the pair are off to New Zealand for a belated honeymoon - courtesy of his new, millionaire son Gary who Celia has come to terms with. And not just because he's bought them a fancy holiday.
Played by: Derek Jacobi, who's best know for his hugely successful theatre career. On TV he stars in Vicious with Ian McKellen and you'll have seen him in Gosford Park and The King's Speech.
Caroline (Sarah Lancashire)
Celia's daughter Caroline has had quite a time of it. In series one she's married to Tony (he's pretty awful) who is cheating on her while she is cheating on him with a woman named Kate. Also it's important to know that when Caroline meets Alan's daughter Gillian for the first time, they clash massively. Gillian thinks Caroline is a snob, Caroline thinks Gillian is insufferably cynical and moody. But slowly they start to get on and confide in each other.
In series two, Kate decides she wants a baby and Caroline eventually agrees, even though she's already got two teenage sons William and Lawrence. A guy called Greg becomes the sperm donor and Kate is about to have the baby. But in a shockingly sad episode - one of those where you have to hide behind a cushion because it's so painful - Kate dies in a car crash and her baby is delivered safely. Suddenly Caroline is a widow and a new mother in the space of a few hours. To make her life even more baffling, ex-husband and cheater John suggests they give their relationship another go. She declines. And as series three ends, Caroline is struggling with grief as she talks to Kate's 'ghost' but is absolutely in love with baby Flora, who she is now bringing up alone.
Played by: Sarah Lancashire, who you'll know as Catherine in Happy Valley, Miss Audrey in The Paradise, Raquel in Coronation Street.
Gillian (Nicola Walker)
Now, Gillian makes Caroline look like she's had a quiet life. We first meet Gillian, Alan's daughter, when she's recently been widowed by her husband Eddie. In series one, confused about life, Gillian has a one night stand with Caroline's ex-husband John, who is still involved in the family. It's all very strange- but that's the power of a few too many drinks.
John is convinced he's infatuated with Gillian but this fizzles out. Meanwhile Gillian is left looking after her grand-daughter after her teenage son Raff and his teenage girlfriend Ellie fail to care for their baby properly. And to make matters more complex, she's having a fling with Robbie, her late husband's brother.
In series three, after a turbulent romance, Robbie tells Gillian he wants to marry her but she freaks out and sleeps with John instead. Yep that John, her half-sister's ex-husband. Oh Gillian.
Cheating is definitely a theme in this show...
Despite having doubts and nearly bolting, Gillian does gets down that aisle. So when we see the couple in the Last Tango Christmas special, they're married and living together on the farm.
BUT it's crucial to know this: It's revealed in an earlier series that Eddie's death wasn't entirely an accident and that Gillian killed him after she couldn't stand him abusing her anymore. It's a pitch-black secret that has made her self-destructive, guilt-ridden and confused. And most importantly, that's partly why she felt she shouldn't marry Robbie. She's very much still dealing with the horror of what she did to Eddie in desperation.
Played by: Nicola Walker, who you'll know as Cassie in Unforgotten, Jackie in River, Sharon in Babylon, Helen in Scott & Bailey.
Other crucial characters...
John (Tony Gardner)
Caroline's unlikeable, insecure and self-obsessed ex husband who fell for Gillian - or thought he did. He's now back with his former lover, the suitably terrible Judith.
Played by: Tony Gardner who 90s kids or parents of 90s kids will know as Brian from My Parents are Aliens. But also Dan Miller in The Thick of It, Professor Tony Shales from Fresh Meat.
Judith (Ronni Ancona)
John's former lover, now his partner - children's novelist and top-class snob.
Played by: Ronni Ancona who you'll know from The Big Impression alongside Alistair McGowan - she played Nigella and Posh Spice particularly well...
Robbie (Dean Andrews)
Gillian's husband and former brother-in-law. He's just retired and is driving Gillian up the wall.
Played by: Dean Andrews who you'll know as Ray in Life on Mars.
Greg (Marcus Garvey)
Greg was the sperm donor to the late Kate's baby Flora. Caroline is now a single parent to Flora but Greg helps out despite being a bit of a disorganised mess.
Played by: Marcus Garvey who you'll know as Pete from Broadchurch and Keith in You, Me & Them.
Raff (Josh Bolt)
Gillian's son became a dad as a teenager and has a baby called Emily Jane with his girlfriend Ellie. The pair struggled with parenthood at first but they seem to have it mostly under control.
Played by: Josh Bolt who played the lead in one-off drama Just Henry alongside Sheridan Smith.
Ellie (Katherine Rose Morley)
Played by: Katherine Rose Morley who you'll know as Emma Moxam from recent BBC3 drama Thirteen.