The Dregs are the main antagonists of the Doctor Who episode "Orphan 55". They are the mutated remnants of humanity following a nuclear war and a series of global catastrophes resulting from climate change, who continue to inhabit Earth (now renamed "Orphan 55").
Following several global catastrophes resulting from climate change, mass migration and the collapse of the food chain lead to nuclear war, devastating the planet and causing humanity to mutate into creatures known as "Dregs", which adapted to survive in Earth's new radioactive, high CO₂ atmosphere, retaining their human intellect. Earth was designated "Orphan 55" after this catastrophe.
Despite Orphan 55 being practically uninhabitable, a group of people lead by Kane illegally built the Tranquility Spa, a leisure resort, in the location that used to be Siberia, protected by an ionic membrane to prevent the Dregs from entering the resort.
At an unspecified point in Orphan 55's history, one of the guests, Bella, Kane's daughter, who she abandoned, infected the servers with the hopper virus to get revenge on her, causing the ionic membrane to deactivate and allowing the Dregs to enter. As the Dregs stalked and killed guests, the staff attempted to fight them, but were defeated until the time traveller known as the Doctor, who was at the resort, managed to reactivate the ionic membrane, disintegrating the Dregs inside. However, Benni, one of the guests, failed to enter the resort during the attack and was dragged away by the Dregs.
After discovering Benni's disappearance, Kane, the Doctor, Bella, staff members Hyph3n and Vorm, Benni's fiancée Vilma and the Doctor's friends Ryan Sinclair, Graham O'Brien and Yasmin Khan left to find him with radiation protection. During the hunt, the Doctor figured out that the Dregs were able to adapt to anything, including gunfire. The truck soon broke down, and was swarmed by the Dregs, who had Benni with them, although Kane shot him at his request. Most of the group managed to escape through the bottom of the truck, although Hyph3n and Vorm were both killed.
The Dregs pursued them, but Vilma sacrificed herself by staying behind and being devoured by the Dregs while the others escaped. Later, the Doctor came across several unconscious Dregs, and investigated to learn more about them, finding that they possessed psychic abilities and breathed in carbon dioxide and breathed out oxygen, the direct reverse of the human respiratory system. A Dreg attacked her, but Kane shot at it, allowing her to escape. Kane later decided to remain behind out of remorse.
The group got back to the spa, but were attacked by the Dregs, as a bomb planted by Bella had damaged the ionic membrane. As the humans desperately attempted to refuel the spa's teleporter in order to get off the planet, the Doctor communicated with the Dreg's leader, who she named "Wheezy", pointing out that the only CO₂ in the resort was breathed out by the humans, meaning that the Dregs would suffocate if they killed the humans. The Dreg leader understood, but was killed when one of Bella's bombs went off soon after.
After the teleporter was refuelled, the humans attempted to escape, but found that the teleporter couldn't teleport them all at once, meaning someone would have to stay behind. Bella volunteered, and she and Kane, who had escaped, remained at the spa to fight off the Dregs as the others teleported off Orphan 55. What happened afterward is unknown, but most likely the Dregs killed Bella and Kane.