As Jenny Lee settles into her role, there is a new arrival at Nonnatus House, Chummy (played by Miranda Hart).

A welcome pair of extra hands, Chummy sets about showing her worth, but an overwhelming keenness to please, and her inability to ride a bike, prove obstacles in gaining the respect of Sister Evangelina.

Meanwhile, a chance encounter with Mary, a young pregnant runaway, forces Jenny to see an unfamiliar, darker side of life…