In the year 2000, Joan Stanley is enjoying retirement in suburbia when her tranquil life is suddenly disrupted when she's arrested by MI5 and accused of providing intelligence to Communist Russia. Cut back to 1938 where Joan is a Cambridge physics student who falls for young communist Leo and through him, begins to see the world in a new light. Working at a top-secret nuclear research facility during WWII, Joan is confronted with an impossible question - what price would you pay for peace?