We all know about your secret but we can't let you in on the fact that we know. Now we're here to make sure that you don't find out that we know because we're scared to see what will happen if you know that we know.
Haruhi is a girl who comes to the realization that she's just an ordinary young girl who has nothing special going on in her life, so she decides to do something about it and to try and make her life as exciting as possible. A boy in her class called Kyon gets dragged along in her shenanigans and this is where Kyon meets Koizumi, Mikuru and Yuki who all have their own crazy supernatural stories to tell.
I've personally not read the source material but I do get the feeling that the story is told in an inaccurate order due to some of the early events that happen in the series. It's pretty clear that Haruhi wants to impress Kyon but she really struggles to be direct open with him while he is still on his journey to figure out how he really feels but it's clear that they are both on this adventure together.