The Weasel is the secondary antagonist of the 2012 sci-fi comedy film Men in Black 3. It is an extraterrestrial anthropoid serving as the sidekick of Boris the Animal on his plot to deactivate the Arcnet Shield to allow his fellow Boglodites perform an invasion and feed on Planet Earth.
On July 15, 1969, Boris has the Weasel to kill Roman the Fabulist whilst searching for the ArcNet Shield at Coney Island, according to the MiB Archives. Later, at Cape Canaveral where Agent K placed the ArcNet Shield on the Apollo 11 rocket, Boris attempted to stop K from doing so, only to have his arm blown off and sent to LunarMax Prison. K regretted not killing him for many years following the incident. The establishment of this shield around Earth prevented the Boglodites from entering, parasites that strip a planet of its resources, thereby leading to their extinction. This only fortified Boris's grudge against Agent K. However the Weasel somewhat escaped from being caught and remained hidden for the next 40 years.
Plotting for Revenge
After almost 43 years, the Weasel turned over to Lilly, who brought it over to to the LunarMax to free Boris. After killing two guards, the Weasel reunites with Boris, who then proceeds to kill the remaining guards, Lily and an inmate named Obadiah Price.
After a close encounter with both K and J at the roof of a Chinese restaurant, Boris resolves to kill K instead by travelling back in time -time jumping- to the year 1969 to stop K from forcing his race into extinction and destroying/shooting off his arm. In the altered timeline, the elder Boris and the Weasel travel back to this year with the aim of working together with their past selves to kill K and destroy the ArcNet to ensure their species' survival.
The younger Boris manages to track down Griffin, the rightful possessor of the ArcNet Shield, and orders the younger Weasel to steal the box seemingly containing the shield. However, in a brief encounter with the young K and J, they manage to rescue Griffin, and Boris and the Weasel are forced to escape, later learning that the box is empty. However, both the young K and J need to install the ArcNet in the Apollo 11 at Cape Canaveral (with the help of an Army Colonel named James Darrell Edwards II, who happens to be J's father) before it blasts off into space. The two Borises, along with their Weasels, follow the agents to Cape Canaveral, where the elder Boris fights J while the younger Boris tries to stop the younger K from installing the ArcNet into the rocket. Just as the elder Boris attempts to kill the young K, J calls him by his nickname, thus causing him to get distracted and annoyed as he orders his Weasel to fire his spikes against agent J.
J is then pelted with three-four spikes before pushing himself, the elder Boris and the Weasel off the launch tower. Eventually, J time-travels himself back to where he faces off the elder Boris, however, this time he remembers where the spikes hit him and uses this to dodge the spikes instead, much to the elder Boris's shock. This allows J to punch off the elder Boris and the Weasel off the tower. Around the same time, the younger Boris's left arm is still destroyed by K when he uses one of the fueling hoses containing liquid nitrogen to freeze his arm and shatters it by shooting it. Following the successful installment of the ArcNet, the agents escape from the tower as Apollo 11 begins to lift off, and the elder Boris and the Weasel are both incinerated by the engine exhaust from the Saturn 5 lift vehicle. As the younger K reunites with Colonel Edwards, the younger Boris arrives, having escaped the tower, and gets his Weasel to launch a spike on K, only for the Colonel to receive it himself, an act witnessed by J (who was shocked).
Following the Colonel's death, the young K holds the younger Boris at gunpoint, and Boris, revealing his true form, attempts to goad K into arresting him, so that he can start the cycle again. However, the young K refuses to do so, and instead shoots the younger Boris, killing him and the Weasel in a blast. This erases their older selves from history, also avenging the deaths of the humans and the aliens they had killed. With Boris and the Weasel finally dead, the parasitic Boglodites starve themselves into extinction over the next 40 years, as they can't survive without feeding off resources from a planet.