A prototype aircraft called Project Excalibur, capable of deploying nuclear weapons, has been stolen from a secret British airbase in Scotland. The aircraft is now located in the country of Rushmania and poses a threat to UK security. Two flights of the British Special Air Service (SAS) squadron 318, codenamed Arthur's Crown and Merlin's Staff, are deployed on a mission to recover or destroy Excalibur, eliminate intelligence on the project, and extract without being caught to maintain government deniability.