The new 2012 Men In Black III released in May, Agent J (Will Smith) travels in time to MIB's early years in the 1960s, to stop an alien from assassinating his friend Agent K (Tommy Lee Jones) and changing history.

From Wikipedia:  Men in Black (MIB), in American popular culture and in UFO conspiracy theories, are men dressed in black suits who claim to be government agents who harass or threaten UFO witnesses to keep them quiet about what they have seen. It is sometimes implied that they may be aliens themselves. The term is also frequently used to describe mysterious men working for unknown organizations, as well as to various branches of government allegedly designed to protect secrets or perform other strange activities. The term is generic, used for any unusual, threatening or strangely behaved individual whose appearance on the scene can be linked in some fashion with a UFO sighting.