Lieutenant-Commander Lionel Kenneth Phillip Crabb,, known as Buster Crabb, was a Royal Navy frogman and diver who vanished during a reconnaissance mission for MI6 around a Soviet cruiser berthed at Portsmouth Dockyard in 1956.

The Secret Intelligence Service (SIS), commonly known as MI6, is the foreign intelligence service of the United Kingdom, tasked mainly with the covert overseas collection and analysis of human intelligence on foreign nationals in support of its Five Eyes partners. SIS is one of the British intelligence agencies and the Chief of the Secret Intelligence Service ("C") is directly accountable to the Foreign Secretary.