While watching Memento, it is quite difficult to construct film's fabula because of the film's backward structure. The main reason is that facts are presented neither in chronological order, nor following the casual relationship. In addition, it is hard to provide a coherent version of the story, since at the end of the film we touch the unreliability of the main character's recollections. .
From the beginning we have to grasp the film structure and structure of actions. It is not as difficult to place narrative elements into a coherent structure, but there is a sense of difficulty in using our own memory. The problem is that when recalling a part of the film, we are not able to place a fact after another, because we cannot remember if it was before or after; i. e. if we should place it in the structure of the film or in the film's fabula. In other words, the operation of mental rotation is not simple. We can do it at a point in time, but not for the whole range of the story (because of the capacity of our short-term memory). Each time we need a mental representation of the fabula, beginning from the end, and we have to consider a great number of elements. While the film continues to present new elements to be processed, it's almost impossible to keep the order of fabula structure in mind. In order to understand the film, we have to be able to control the two different schemata: the fabula and the syuzhet.
Short summary of the film's fabula that we figure out as we go: Leonard Shelby is a former insurance investigator. In his previous life, intruders rape and kill his wife one night. He kills one of them, but the other bonks him on the head and gets away. The injury leaves him suffering from a condition called anterograde amnesia, which means that he can't create new long-term memories. Leonard can remember everything prior to the accident, since his old long-term memories are still intact; but his current attention span lasts roughly 15 minutes (and even less when he's stressed or distracted), and in no case can any of these current memories be permanently implanted in his brain.