From the Drain (1967)

The film is centered on two men in a bathtub; it is implied that they are veterans of some past conflict but revealed that they are currently in a mental institution. The first man is paranoid about the drain of the tub, the second indifferent to it. After the conversation between the two men progresses, a vine-like tendril emerges from the drain to strangle the first man. The second shows no emotion to this sudden turn of events and the film ends.