Complex sentence: To celebrate, Marty goes trick-or-treating, but although he is ostensibly just trick-or-treating, he is also looking for a man or a woman missing his or her left eye.
Split into simpler sentences: To celebrate, Marty goes trick-or-treating. Although Marty ostensibly wants to go trick-or-treating, he is also looking for a man or a woman missing his or her left eye.