In this funny, nuanced sci-fi parable, urbanites Derek and Mari face the arrival of extraterrestrials who challenge their patience and their comprehension. "We come in peace," the humanoids announce, and this claim appears to be accurate. But who are these aliens, and what do they want? Their mission on Earth soon seems both harmless and threatening. Arriving without fanfare or commotion, these creatures -- the so-called Missionaries -- do no harm toward earthlings. They are in fact altogether gentle, non-violent, unassuming, and cordial. They look like ordinary human beings and go about their business without causing trouble. But Derek and Mari, like all humans, wonder what that business may be. When two aliens repeatedly show up on the couple's doorstep, it appears that their only intention is to "witness" about their religion. Derek is initially skeptical, then uncomfortable, then hostile to the Missionaries' intrusions. Mari, by contrast, is curious at first and soon intrigued. Tensions flare between the spouses. When the alien pair -- Herbert and Agnes by name -- press their agenda more aggressively, Derek and Mari find their twenty-year marriage under stress. Mission soon portrays how the biggest threat they face isn't violence from outer space but, rather, their own altogether human lack of mutual comprehension.