Michele Wilkes, an Emergency Medical Technician, races an ambulance through the streets of Philadelphia as her highly-regarded partner, Leonard Betts, attends to a dying man. As the ambulance speeds through an intersection, it is broadsided by a truck. Wilkes survives, but she discovers Betts' decapitated body nearby.

Mulder and Scully are brought into the case when Betts' headless corpse disappears from a hospital morgue. Scully theorizes that someone attempted to steal the body and sell it to an unscrupulous medical supplier. Securitv camera footage shows an unidentified person with a strange distortion around the head area leaving the hospital sans Betts' body. With Mulder's reluctant help, Scully fishes through the hospital's bio-disposal unit for the missing corpse. The agents recover the missing head, but the whereabouts of the body remains a mystery. As Scully performs an autopsy on the head, its eyes and mouth suddenly snap open.

A tissue sample taken from Betts' head is analyzed by a pathologist who determines that Betts' body was riddled with cancer. Mulder hypothesizes that Betts possessed unimaginable regenerative powers--perhaps the next step in Man's evolution. A fingerprint search reveals that Betts' real name is Albert Tanner. The agents interview his mother, Elaine, who claims that Albert died in an automobile accident six years earlier.

As Wilkes is transporting another patient, she recognizes Betts' voice transmitting over the ambulance radio. She locates Betts outside a Catholic hospital. Without warning, Betts pulls out a syringe and injects her with a lethal substance. A security guard witnesses the murder and handcuffs Betts to his car in which he attempted to flee. Betts slips out of the cuffs by tearing off his thumb.

When the agents search the trunk of Betts' car, they discover plastic bags filled with human tumors. Mulder postulates that Betts is ingesting the cancerous tissue in order to survive. They learn that the car is registered to his mother, who claimed not to know about Leonard.

Betts' advanced powers allow him to identify individuals who have tumors within their bodies. He attacks and kills a heavy-smoking bearded man after he leaves a bar. Afterward, Betts "gives birth" to a duplicate of himself.

The agents discover a storage locker key at Mrs. Tanner's home. When they open the door to the locker, the bearded man's body, its left lung missing, rolls into view. Using the bearded man's car, Betts attempts to run down Mulder and Scully. The agents dive for cover and open fire. The car bursts into flames, incinerating Betts.

Mulder tells Scully that Betts had ingested the bearded man's cancerous lung. Scully reminds her partner that Betts is quite dead and his not coming back to life. Mulder exhumes the body of the man Mrs. Tanner claimed was killed in the automobile accident. Inside the casket is a corpse identical to Betts. Mulder concludes that the fiery crash was a ruse-and the real Leonard Betts is still alive.

The agents stakeout Mrs. Tanner's home. When an ambulance arrives at the scene, Mulder and Scully race inside. They find Mrs. Tanner, still clinging to life, with a fresh surgical incision on her chest. Scully accompanies Tanner to the hospital. When the ambulance pulls into the hospital parking lot, she realizes Betts hitched a ride by hiding on the vehicle's roof. Betts jumps inside the ambulance and slams the rear doors behind him. Wielding a scalpel, he tells Scully she possesses "something I need." A scuffle ensues, and Scully kills Betts using defibrillation pads. Later that night, as Scully lies in bed, she wakes up coughing and finds a few drops of blood on her pillow. Blood drips from her nose, and she recalls Betts' haunting words.

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