MEN IN BLACK 1 et 2

In 1978, a race of aliens came to Earth and asked the MIB to help protect their greatest treasure which they call "the Light" which is a source of awesome power. But it's not in their mandate to do that so they tell them they can't and send them away. But in the present day, Serleeena an alien who wants the light comes to Earth because she suspects that the MIB did hide it. So she goes to MIB headquarters and takes it over. Zed tells J, that he has to get K who was the one who dealt with the aliens in 1978, to come back. But problem is that K's memory has been erased so when J tells him, he doesn't believe him but J eventually convinces him. So they head back so that he could be deneuralized but since Serleena took over the HQ where the deneuralizer is, they have to find another one. They eventually do and restore his memory but problem is K doesn't remember what happened to the light. But because of some clues K planted, they try to figure out what happened to the light.