An old friend of Sebastian Cabot comes racing into Checkmate's office/living quarters with three men with guns on her heels. They're a paranoid bunch who think they may have accidentally spilled some criminal plans. Fortunately Anthony George is at home and awake. It's only when Cabot arrives on the scene that they can 'talk' to her. Lynn is not only deaf, she's a teacher of deaf kids.
As it turns out she does read lips fluently and did catch some words while at a coffee shop with these three gorillas. Cabot who is fluent in sign gets some of those words from her. He puts them down on a black board and tries to figure out their meaning in a scene reminiscent of the later The List Of Adrian Messenger with George C. Scott figuring out what the Adrian Messenger character was saying in his dying words.
Paranoia is what does these three in. Had they just ignored her, their nefarious scheme would not have come to light. What was their scheme. Not saying more, but think about the plot premise of Key Largo.
Diana Lynn is quite good as the deaf mute teacher.