The premise - A man's life forks when he's attempting to board a train, and the movie traces each possible destiny from the fork. That is the plot. It's set against Communist Poland, and as such is worthwhile just for the reminder of how devastating the Soviets were to culture (and - in different ways, still are today), but it has some of Kieslowski's signatures, uncomfortably intimate moments, atmospheric, haunting.
Overall, my least favorite of his movies. Which is to say it's a damned good film by any standards, but what followed was breathtaking. If you're just discovering him maybe view the Dekalog first, then go back to the first two, then forward to the Blue, White & Red trilogy & La Double Vie de Veronique.