The neutron eaters fed on the neutrons in normal matter. This diet would sicken and weaken humans who were fed on it, reducing their density and causing them to give off an unusual glow. If the neutron eater was destroyed, the neutrons would be returned, instantly healing the victim.
When CERN's Large Hadron Collider was switched on for the first time, a portal was opened to another reality. Though it was only on for nanoseconds, a neutron eater escaped into the universe. It met Oliver Harrington and convinced him that it was his dead wife Marie. Oliver hid the bodies of the neutron eater's victims. He was convinced that they were turning into angels and they would draw their souls from heaven, returning them to life.
When UNIT came for the official unveiling of the Large Hadron Collider, Martha Jones discovered that people had gone missing. She contacted Torchwood and convinced them to help her investigate. They linked the disappearances to the tunnels of the Hadron Collider. Ianto Jones and Gwen Cooper went to investigate. Ianto encountered the neutron eater, who convinced him that it was Lisa Hallett and then fed on him.
Gwen tried to escape with Ianto. The neutron eater failed to trick her. In the meantime, Jack Harkness tried to convince Professor Johnson to shut down the machine, lest it open another portal and release more neutron eaters. Jack also tried to make Oliver believe that the neutron eater was dangerous, but Oliver didn't believe him and went to confront the neutron eater in the tunnels. Oliver and the neutron eater were trapped in the tunnels and destroyed when the machine was modified to stop the portal from opening.
- When and where this occurred in unknown, but it probably was an Earth colony planet.