When a rampaging alien called a METAPHITE breaks out of its cocoon and threatens Phoenix, MiB is called in to stop it. Unfortunately, all efforts prove useless against the giant alien that is basically a living nuclear reactor. The only solution is to use a "Time Jumper' to travel back one hundred and thirty years in the past when the Metaphite cocoon is vulnerable. Upon arriving smack dab in the middle of the wild west, Kay and Jay locate the cocoon only to be ambushed by another alien, a VEXRON, which grabs the Metaphite cocoon and escapes. Kay and Jay (soon joined by Elle) must race to save the future by figuring out who of western town folks is actually the Vexron in a human suit. Along the way there's an old-fashioned "high noon" gunfight, a barroom brawl and a prison break out. In a climatic chase, our agents thwart the Vexron and capture the Metaphite, just in the nick of time to save the future.