In Star Trek: The Motion Picture, the mysterious cloud threatening Earth is called V'Ger.
The enormous energy cloud appears in 2270 and destroys three Klingon vessels before heading toward Earth. Starfleet sends the newly refitted USS Enterprise to investigate because the cloud is on a direct course for the planet.
V'Ger is eventually revealed to be a transformed Voyager 6 probe. The probe was launched from Earth centuries earlier, then encountered powerful machine entities that enhanced it and returned it to its creator. Its original designation had become difficult for humans to recognize, leading to the name V'Ger.
The film's central conflict comes from V'Ger's search for its creator and its inability to understand human individuality. The probe's identity is sometimes confused with a conventional alien character, but V'Ger is primarily a vast machine intelligence and energy phenomenon. The 1979 film was the first Star Trek feature released after the original television series.