Nicola Jane Walker is an English actress, known for her starring roles in various British television programmes from the 1990s onwards, including that of Ruth Evershed in the spy drama Spooks and DCI Cassie Stuart in Unforgotten (2015–2021). She has also worked in theatre, radio and film. She won the 2013 Olivier Award for Best Supporting Actress for the play The Curious Incident of the Dog in the Night-Time and was twice nominated for the BAFTA TV Award for Best Supporting Actress for the BBC drama Last Tango in Halifax.

Spooks is a British television spy drama series that originally aired on BBC One from 13 May 2002 to 23 October 2011, consisting of 10 series. The title is a colloquialism for spies, and the series follows the work of a group of MI5 officers based at the service's Thames House headquarters, in a highly secure suite of offices known as The Grid. In the United States, the show is broadcast under the title MI-5. In Canada, the programme originally aired as MI-5, but later aired on BBC Canada as Spooks.