Mulder and Scully investigate a series of attacks by a canine that shows signs of human intelligence.
On a freighter in the South Pacific two Chinese men are inspecting a crate with an animal inside. They open the box. When the ship reaches port the authorities find the two men in the crate and the animal missing.
In Mulder's office, he explains to Scully the incident on the ship. He hands her the report that says the animal was a dog and that the crate was locked from the outside. Also that the men died of bite marks but Scully says that the only way to die from a dog bite is to bleed to death which these men did not.
A man hears his dog barking in the backyard and lets it in the house. He runs the other dog off and returns to the house where he finds his dog dead. He then hears the growl of the other dog as it enters the room. The dog then shows its teeth and its eyes glow red before it attacks him.
Mulder and Scully arrive at the port and talk to the owner of the dog Dr. Ian Detweiler, a cryptozoologist, who says the dog is an Asian breed thought to be extinct for 150 years. After investigating the other crime scene Mulder believes the dog has near human intelligence.Mulder and Scully visit Karin Berquist, an expert on canine behavior, who the breed they are looking for is extinct. Meanwhile a dog catcher is chasing the dog through a warehouse when a man enters the hallway and comes closer. As he gets near he transforms back into the dog and attacks the dog catcher.
Mulder and Scully arrive at the scene followed by Berquist. She finds a paw print with five toes, one more than regular canines. After words Scully questions her motives and believes she has a thing for Mulder.
Detweiler arrives at a vet's office to get tranquilizers and leaves. The vet goes to the back and is attacked but he locks the dog in the back room. The dog catchers arrive and open fire on the wrong dog. The vet finishes patching up the hurt dog as Scully enters the building. She hears a scream and finds the vet has been attacked. After Scully leaves the hurt dog transforms back into the Asian breed.
Scully questions Berquist about her being sick and her luring Mulder into this investigation. Mulder is searching the vet's office and discovers Detweiler had been there and calls the head wildlife officer. The officer gets in his truck and is attacked. Mulder visits him in the hospital and finds Detweiler already there. Mulder confronts him about being the killer and that he tried to tranquilize himself to stop it. Mulder visits Berquist and says he thinks she has not been honest with him. She said she knew when she saw him he was the animal. She says he will try to finish his kill. Mulder explains his theory to Scully but she is skeptical and doesn't believe Berquist.
Berquist is locking the outside kennels at her house when she hears something in the woods. She goes upstairs and gets a tranquilizer gun. Mulder figures out she lied and she is the next kill not the officer. He calls her answering machine and tells her to lock the doors and that he is on his way.
The dog enters Berquist's bedroom and attacks her and they both fall out the window. Mulder and Scully arrive and find Detweiler had fallen on a spike and Berquist dead. Back at the office Mulder blames himself. Scully then hands Mulder a package from Berquist. It is her I Want to Believe poster and Mulder hangs it up.
- Mulder receives a new copy of his "I want to believe" poster. His original was destroyed in "The End".
- Bellflower, California, one of the episode's settings, is series creator Chris Carter's hometown.
- Melinda Culea, who plays the role of Karin Berquist, is the wife of the episode's director, Peter Markle.
- T'ien Kou, the name of the Chinese freighter, translates to 'heavenly dog', a Chinese term for 'shooting star'.
- This episode has an alternate ending. In that, the episode doesn't stop with Mulder looking at the poster, but with Cahn turning around in his bed, to reveal that he was infected by the Dhole. This can be seen on the DVD box.
- The vet's kennel door only has a deadbolt in the shot where the fish and game officer opens the door; the deadbolt is missing from all other shots.
- Melinda Culea as Karin Berquist
- Thomas Duffy as Jeffrey Cahn
- Michael Mantell as Dr. James Riley
- David Starwalt as Frank Fiedler
- Andrew J. Robinson as Dr. Ian Detweiler