Frustrated by her girlfriends' constant disgust with her work, Ida meets Misty who keeps to herself and draws countless sketches of pixies. As Ida becomes more attracted to Misty, she receives an unusual package of a bizarre mantis-like insect she later names "Mick". Tensions only worsen as her landlady becomes disdainful of how her granddaughter looks up to Ida and "Mick" escapes its cage after Ida leaves.
Ida and Misty eventually come together as Mick makes itself comfortable in one of Ida's pillows (after devouring the landlady's dog). As Misty sleeps over using that pillow, Mick bites her ear. From this point on, Misty demonstrates unusual tendencies such as eating stray insects and becoming much cruder. Ida consults her fellow entomologist Max about Mick but is dismissed. Mick eats from Misty's ear and inserts its proboscis into her ear. Ida receives distressing news from the sender concerning how dangerous Mick's species is. Misty crudely insults the landlady which leads to Ida being threatened to leave her apartment in one week. When confronted, Misty explodes at Ida and retreats into the apartment.
Max finds information about how Mick's species has a disturbing tendency for drinking its hosts blood and inseminating them to carry its offspring via DNA corruption. When the landlady posts the eviction notice against Ida's apartment, Misty insults the woman. Disgusted, the landlady calls her a monster. On cue, Misty suddenly sprouts insectoid eyes and multiple tendrils. Her transformation causes the landlady to fall to her death. As her granddaughter cries over the death, Misty coldly dismisses the affair which angers Ida.
To make things worse, it is revealed that the sender was Professor Wolf, an idol of Ida's who had sent the bug in order to make Ida repulsive to Misty who had long been attracted to her. Misty undergoes another transformation into a monstrous bug hybrid and goes after Ida. Max arrives and tries to help, only to be killed by Misty. During the murder, Mick appears and begins the insemination process with Ida. And thus Sick Girl ends with Mick continuing to imseminate the two women who will most likely be killed once the babies are born.