Our main character, Tony (Jim Broadbent), is a divorced Londoner in his 60s running a very small camera shop. He's on reasonably good terms with his ex-wife, and their daughter is about to have a child. Into his fairly quiet life is dropped a small sum of money left him by the mother of a girlfriend he had in university, along with the mention of a diary that had also been left to him. The diary is in the possession of the ex-girlfriend, who he attempts to contact.
Tony initially seems like an okay guy - a bit short-tempered, a bit of a prat, but a decent enough person. But as the movie progresses, we see some of his university years in flashback - and watch as he realizes that he wasn't as nice a person as he remembers being. As his behaviour towards the ex-girlfriend in the present day veers into decidedly indefensible territory, he becomes a very unappealing person to be spending a movie with.
But somehow in the last moments of the movie this all comes home to him and suddenly his behaviour towards his own family becomes much better. It feels totally unjustified and untrue to the character we've just spent an hour and a half getting to know. I get that recovering a piece of your past like that might change your behaviour, but I didn't buy the scale of his recovery.
The movie is meticulously constructed and well acted, but I didn't buy the character twist at the end.