The characters all agree to meet at the same time and place, although most of them do not know each other and therefore confusion ensues. There is the aspiring filmmaker who plans on staying with his sister's friend, whilst he studies film making, but ends up going home with a prostitute instead. She was supposed to meet an uptight young businessman who is still a virgin due to his anxiety and repressed feelings regarding his sexuality. He ends up spending the time with another man that he meets at the square. Meanwhile, the filmmaker's sister's friend, who is an aspiring actress who is desperate to land a role, throws herself in front of a famous director's car and then tries to blackmail him into giving her a part in his next movie. Another story features a middle aged women, who hires an escort, only to come to believe that he may be her long lost son that she was forced to abandon shortly after birth.
There are more sub-plots but each one is more boring and less interesting then the ones above. Pretty much all of the stories revolve around contrivances big enough to blot out the sun on the hottest day of the year in Madrid - which is when this story is supposed to be set. The only two stories I was able to develop any interest in were the one with the filmmaker trying to improve the life of the prostitute, and the businessman, who is so uptight and repressed, that he's willing to consider a dalliance with a whore, even though he is engaged to soon be married. The others failed to affect me at all.
The plots feature both straight and gay couples, with practically everyone being paired off at the end in some manor. The quasi-happy, yet completely unbelievable, ending was probably where the film fell apart altogether. In spite of the contrivances and plot holes, I was willing to give the film a certain amount of latitude, but the ending destroyed any good will that it had built up.
I'll admit the acting is uniformly good, with completely naturalistic performances all around. There are also plenty of reviews out there that describe Km.0 as "charming" and "funny". However, whatever charming and funny moments the film has are vastly outweighed by it's numerous flaws. If you don't mind the subtitles and ridiculously contrived plots, then you might enjoy it. Otherwise, I would think it's best if one kept ones distance from this film far greater than zero kilometers.