It's a story that's difficult to summarise, but basically a rather silly 21-year-old failed poet hires an 'Existential Detective Agency' (Dustin Hoffman and Lily Tomlin) to uncover the meaning of his life. Their approach is that everything is inherent with meaning, and connected to everything else. I'm simplifying. It turns out that there is a rival detective - a sexy middle aged french woman - who represents the nihilistic aspect of existentialism, and believes that everything is meaningless and cruel. She seduces the poet.
Are the two detective agencies rivals? Enemies? Aspects of the same detective agency? Duh, is this an allegory or what?
There's a bunch more there, and a lot of other characters, including Jude Law, who is probably stretched to the limit of his range a couple of times, but hangs in there and gives it his best shot. There is a scene where the detectives play him a recording showing that he has told the same self-satisfied anecdote, about Shania Twaine, over and over again. And Jude's face sort of collapses from amusement to self disgust over several minutes. Well done, pretty boy, I knew you could do it. Now have a rest.
There is a superb climax to this story-line by the way (sorry this is a spoiler) when Jude meets Shania Twain in a lift, and she punches him in the face. Yay. 'It don't impress me much.'