Informational film about organisms invading the blood. Shot of the culture medium undergoing changes. It looks like a bright blob of light. The white blood cells (Leukocytes) are spreading. It looks like a sun. Shot of white blood cells. Narrator states this is what happens inside the body when an organism invades it. Single leukocyte looks like an amoeba. It is a jelly-like blob of protoplasm. The cubicle bacillus is on the screen. The leukocyte slithers up to the cubicle bacillus and gathers around it.