Mason Bennett
Mason Bennett was the marine geologist onboard the Drum, an underwater mining facility in 2119 Scotland.
When commanding officer Jonathan Moran was killed, his friend Cass took charge. She led the remaining crew to the base's Faraday cage, where they would be safe from the ghost leader Albar Prentis and Moran's newly-created ghost.
When the Twelfth Doctor decided to travel back to before the flood to uncover the mystery behind the ghosts, Bennett and Alice O'Donnell were the only ones able to get back to the TARDIS with him, and the two journeyed with him back in time. (TV: Under the Lake)
Arriving in Caithness before the town was flooded, Bennett and his friends explored the Fisher King's spaceship before the markings had appeared. They also met with Prentis not long before his death.
Later, when O'Donnell was killed by the Fisher King, Bennett and the Doctor retreated back to the TARDIS and attempted to go back to 2119, only to find out that they had traveled to 30 minutes earlier. While watching themselves speak with Prentis, Bennett wanted to speak to O'Donnell again but the Doctor pulled him down, assuring him that he couldn't do it or he would cease to exist.
Later, when the Doctor faced the Fisher King, Bennett went back to the TARDIS and was sent back to 2119 by one of the Emergency Programs.
After the crisis, Bennett, Cass and Lunn were the only ones left alive. As Bennett stared at O'Donnell's ghost in the Faraday cage, he admitted that he had loved her, and gave Lunn a message to translate to Cass to say that he (Lunn) loved her but was too nervous to admit it. (TV: Before the Flood)
