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.