Cass (Under the Lake)

Cass was second in command of the Drum, an underwater mining facility in Scotland. She was also deaf, and used her good friend Tim Lunn to translate her sign language.

When commanding officer Jonathan Moran was killed, Richard Pritchard took charge. She led the remaining crew to the base's Faraday cage, where they would be safe from the ghost leader Albar Prentis.

After Pritchard's death, Cass was recognised as the leader of the team. When the Twelfth Doctor arrived, he hatched a plan to trap the ghosts. However, Cass and Alice O'Donnell stayed back to man the controls. Cass was able to lip-read the silent words that the ghosts kept repeating in unison, which proved vital to locating an alien artifact.

When the Twelfth Doctor decided to go back in time to before the flood to solve the ghost mystery, Cass, Lunn and Clara Oswald became trapped behind the flood doors. The three retreated into the dining area, where Cass expressed her doubt the Doctor would rescue them. They then caught sight of a glowing figure outside in the water only to stare in horror at the sight of a ghostly Doctor. (TV: Under the Lake)

After the threat was ended, Lunn, Cass and Bennett were the only survivors of the base personnel. Looking in on O'Donnell trapped in the Faraday cage with the other ghosts, Bennett had Lunn translate a message from him to Cass that revealed that Lunn was in love with Cass and how Bennett wished he'd told O'Donnell before she'd died that he loved her. Cass is shocked by Lunn's confession and he quickly apologizes before she shocks him by passionately kissing him. (TV: Before the Flood)

Behind the scenes
Cass (Under the Lake)
 * Cass is the first deaf character to appear in Doctor Who. Sophie Stone is not the first deaf actor to appear on the show, however. Tim Barlow, who played Tyssan in Season 17's opening four-part story Destiny of the Daleks, was profoundly deaf at the time of filming, but was able to lip-read.