The Human Centipede: A crazy surgeon sews three people together ass to mouth just to prove that he can.
The Human Centipede II: A mentally disturbed garage attendant staples ten people together ass to mouth after watching the The Human Centipede.
The Human Centipede III: A fascist prison warden decides to sew 500 inmates together ass to mouth after viewing The Human Centipede II.
I don't have inside information on what the plot of a Human Centipede IV might be, but following the above pattern, I assume it would involve a guy in surgical scrubs sewing 1000 theater-goers together ass to mouth as they wait in line to purchase tickets to Human Centipede III.

