Alice Woodard is a plant breeder who works in a lab that focuses on creating new strains of flowers. While her colleague Bella is failing at creating a hardy plant that will survive even weeks of undernourishment and neglect, Alice and her team have successfully created a flower that requires more care than an ordinary plant but which makes their owners happy. Alice decides to name the plants "Little Joe" in honour of her son and smuggles out one of the plants for him. The Little Joes begin to aggressively pollinate which Alice theorizes is because she has made them sterile. The same day Bella's dog, Bello, goes missing. Chris goes looking for him and accidentally inhales some of the pollen. Later on he takes Alice out and despite her obvious reluctance attempts to kiss her twice. The following day Bella finds Bello in the lab. He attacks her and she insists that he has changed. Chris later tells Alice that Bella is mentally ill and had previously attempted suicide before being forced on a year long sabbatical, returning only shortly before Alice began working at the lab. Alice later learns that Bella had Bello put down. Bella tells Alice that the changes to Bello were due to the plant. Alice's son is accidentally pollinated by the plant and begins to act strangely, sneaking his classmate Selma, into the lab and stealing a Little Joe. He later tells Alice that he is considering moving in with his father, Ivan. Bothered by her son's behaviour Alice begins to examine test footage of subjects who have been exposed to the pollen. In every case their family members report that they are acting strangely and have seemed to change since the pollen test. However just as Alice begins to believe Bella's suspicions, Bella is exposed to the pollen herself and dismisses her previous beliefs as paranoia due to her mental issues. Joe reveals that he and Selma stole the plant in order to pollinate Ivan confirming Alice's suspicions her plant carries a virus, especially as she has used unorthodox methods to create Little Joe. However this turns out to be a joke as Chris previously talked to Joe about Alice's concerns. At work Bella reveals that she never inhaled the pollen and was only pretending to be happy in order to blend in with the others; she later tries to commit suicide. After Alice's boss dismisses her concerns she takes matters into her own hands and decides to kill the Little Joes before they are commercialized, lowering the temperature in the lab. She is stopped by Chris who, in trying to prevent her from harming the plant knocks her out on the floor of the lab, exposing her to the plant pollen. Later Alice learns that Little Joe has been nominated for an award meaning that the plant will be sold worldwide. When Chris apologizes for hitting her she kisses him, and later dismisses her concerns as paranoia. She allows Joe to move in with his father and starts a new, happier life with her own Little Joe.
The review aggregator website Rotten Tomatoes reported that 66% of critics have given the film a positive review based on 104 reviews, with an average rating of 6.23/10. The site's critics consensus reads: "Little Joes unorthodox approach may baffle horror fans lured in by its premise – but like its title character, the end result exerts a creepy thrall." On Metacritic, the film has a weighted average score of 60 out of 100 based on 25 critics, indicating "mixed or average reviews".
