The Spy is a stealth unit used by the Allied forces to gain advantage. This allows him to sneak past enemies and into enemy structures, providing substantial benefits. The Spy's action inside the enemy structure depends on which building he enters. Attack Dogs are never fooled by the spy's disguise. Watch out for the Attack dogs & mind control units.

If a Spy enters in:
Enemy Barracks: your infantry units are veterans.
Enemy War Factories, your vehicles are veterans.
Enemy Refineries, he steal money currently in that Refinery.
Enemy Power Plants, he temporarily shuts down the enemy power supply.
Enemy Radar Domes, he resets the shroud for the enemy.