A SIR Unit is a robot given to help Invaders with their missions to conquer their assigned planet. However, when a SIR Unit begins to malfunction, it behaves in an aberrant manner ranging from simply eccentric to downright demented, and is no help at all to its master or mistress.
GIR is the best-known example of a malfunctioning SIR Unit as he appears more than any other in the series. However, it is in question as to whether he can truly be considered a typical SIR Unit; since theAlmighty Tallest hate Zim, and didn't want to give him a fully functional SIR Unit, they created GIR out of spare parts.
In keeping with his composite structure, GIR rarely acts like a regular SIR Unit, and consistently displays erratic, hyperactive behavior and a complete lack of obedience. The only time GIR ever acts normally is when he is in "Duty Mode," a phenomenon during which one of the many loose wires in his head presumably connects, and he becomes fully functional.
On one occasion, Zim actually succeeded in locking GIR into Duty Mode, only to eventually realize that he had made a huge mistake: While GIR proved to be far more intelligent, independent and effective than his normal self, he also became exceedingly violent and resentful, and - as a by-product of SIR Unit programming to eliminate any threats to the mission, GIR ultimately identified Zim himself as a threat and attempted to eliminate him.
A package of malfunctioning SIR Units was supposed to be sent out to Zim, but the labels got switched, and Invader Tenn received the destructively psychotic SIRs, while Zim received a Megadoomer X-3 Combat Stealth Mech; the last we see of Tenn, the SIRs are destroying her base.