Improbable Mission (also known as Mission Impossible) was a game played on both the British and American editions of Whose Line.

The game involved three performers. One was the voice on the tape (originally Steve Frost, classically Greg Proops, and occasionally Wayne Brady), with the other two (invariably Ryan Stiles and Colin Mochrie) taking the role of the secret agents. The audience provided a mundane task for them to do - such as cutting the lawn or doing the laundry.

The voice performer would then deliver the mission to the agents, who would then carry out the task in as over-the-top a manner as possible, complete with sound effects from spy films and television shows.

Occasionally, the game was played with all four performers. The extra player took the role as an additional secret agent. The four-player version was only played two or three times, and on one occasion, Colin took the role of voice on the tape.

Greg's work as the voice was particularly notable (even when the agents rewound and fast-forwarded the tape), complete with jokes around the "this message will self-destruct" theme, but the main focus of the game was on Colin and Ryan's teamwork. The missions would be completed in highly unorthodox fashion, including frying eggs on Colin's head, skiing on Ryan and dressing up as sheep while manually pulling every strand of grass down. A great many jokes were made about the agents' own appearances, Ryan's height and Colin's baldness being major targets.

The game is also memorable for the "laundry" mission in the American edition, in which the agents had to wash an article of clothing for "the Emir of Groovefunkistan" (Colin referred to him as "the Snackipark of Imar" later on in the mission). Colin consistently made suggestions about using "The Cat" to help the process, even causing Ryan to collapse laughing. The cult status of this particular mission became clear several episodes later, when Colin planned another mission with the words "If only we had a cat!", as well as Ryan prompting Colin with the question "What do dogs chase?" in a later episode to which Colin replied "Jell-O!"

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