Mr. Kang keeps saying there are no aliens and eventually offers money. B only wants $4K and is seen on a security camera from a bank getting that much. Detective Chu puts it together when B pays $3xxx to catch up on his mother's hospital care, and decides to catch him himself and get the honor. He heads off to the rural area and is killed by B.
Detective Chu's replacement, Detective Kim, puts the same stuff together again, tells the squad leader who says they've just caught the guy (wrong guy, of course), but manages just to get caught by B. At this point, Mr. Kang admits to being an alien and tells B why they're on the Earth.
*His 75th ancestor saw a beautiful planet and called it the Green Planet. It was ruled by dinosaurs, but they accidently killed all of them with a virus, so they put people shaped like them on Earth, and we start seeing pictures: the monolith, tower of Babel, Atlantis, genetic changes, atomic bombs, water-covered world with Noah's Ark, baby with monkey face but blue human eyes, then an ape pounding with a bone while we see seconds of wars and recent historical violence, starting with Nazis. So now they tried experiments and have decided to destroy the planet. They have a brief discussion of relativity and B believes him. They leave Kim with a bomb and take Mr. Kang to the factory to set up a discussion with his son, the Prince.
Mr. Kang manages to kill S and think he's killed B but is attacked again when Kim arrives and shoots B. The squad leader turns up, takes all the honor, and we see the police and Mr. Kang getting into a car. Red bolts come from the sky and hit the detectives, then, erm, a UFO descends to take up Mr. Kang who berates his son for not rescuing him earlier. The son says he wasn't called and Mr. Kang says the son should have remembered he can't do telepathy without long hair. This is not the end and be sure to watch through the credits (no need to watch the actual credits unless you read SK ideograms).
So this movie has the results of torture, some almost slap-stick bits, and not quite what you expected at the end. Very weird, and highly recommended.
*I watched this again a few times -- Chapter 10, 1:30-35 -- and still couldn't identify all the wars.