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troperville

A standard television plot. An assignment is received in The Teaser. A planning session is held (Avengers Assemble). Then comes The Caper or The Con, using a Master of Disguise and an A-Team Montage. The resolution is as close to end of The Tag as possible.

At about the 40-minute mark, there is a Pseudo Crisis.

The trope takes its name from the series Mission: Impossible, whose Impossible Mission Force did one of these every week. As such, note that not every impossible mission is an Impossible Mission.

For missions that actually are impossible, see Impossible Task.

Not to be confused with the video game ImpossibleMission. Also see Suicide Mission.

Examples:

Despite its few episodes, Firefly did two of these, in "Ariel" and in "Trash".
Mercilessly parodied in the Whose Line Is It Anyway? game "Improbable Mission," which had agents given missions involving high stakes but ordinary tasks, such as doing laundry or mowing the lawn.
The eponymous task of "Inception" in the film of the same name.
Oddly enough, both Mission: Impossible and Mission: Impossible featured some of these...

Identity Impersonator

Plots

In Another Man's Shoes