Cassandra is a zookeeper and runs into her coworker, Michael, at the zoo. His arm is bandaged and he says that he was randomly bitten by his roommate that morning. Cassandra is slightly nervous about the bite, but continues about her day. She goes to the tiger cages and sees that the big cats are all nervous. She ushers them into their feeding cages and then goes to check what is wrong. As she goes into the closure she smells something rotting and sees a man in the moat surrounding the big cat cage. He looks like a night guard and she asks if he’s ok. He responds with listless movements and a snarl. Suddenly, Cassandra has a realization that this man is dead. She goes to tell her boss what she saw, but he dismisses her. Him and the other guards think that the man in the moat is simply intoxicated or sick and they work to get him out. Cassandra asks the boss to shut down the zoo, but he says no. As she leaves she sees Michael, who looks sick and listless. He begins to run at her and so she runs away and hides in the tiger feeding area. She talks to the tigers when she hears a scream somewhere distant. She grabs a hook of some sort and goes outside to investigate. She sees the man in the moat attacking a woman who is in the tiger enclosure. The man bites her throat as three other guards attempt to get him off of her. They do, but then the woman, who had just died, gets up and attacks the other men. Cassandra is horrified, but doesn’t have time to do anything as Michael comes up from behind and attacks her. She manages to escape, but finds that Michael has bitten her. She realizes quickly that somehow there is an infection that is causing people to die and attack others. She wants to go to a doctor to try and get healed before she turns into one of the dead, but she isn’t sure if this outbreak is contained to the zoo. She creeps outside and sees that the gates of the zoo have been opened and a bunch of the dead are milling about. She walks over to the merry-go-round and sees some of the corpses strapped to the moving horses, spinning round and round.  She decides that if she dies, there will be no one to take care of the animals and so she starts to go around the zoo and open as many of the cages as she can. She starts with the non-predatorial animals and slowly releases many of them. She finally ends up at the big cat cages and one by one lets them free. They all regard her, and escape through the open door. She feels herself slowly being consumed by the disease and so she goes into the lion enclosure and sits outside in the sun.