Synthoids are artificial, programmable replicas of living beings, typically humans. The technology was developed by Cobra Commander and Zartan, using materials they'd obtained from Destro, who is initially unaware of the plot. When the Commander and Zartan demonstrate the technology by creating a synthoid of Destro himself, Destro -- no doubt furious over the Commander's gleeful announcement that the synthoid is more obedient -- orders the Commander to destroy it.
During a staged attack on the GI Joe forces while performing a naval combat exercise for some of the United States military's top officials, Cobra Commander and Zartan -- posing as one of the officials -- successfully replace the top officers observing the exercise with synthoids. When the exercise is over and despite the Joes routing Cobra forces, the officers reprimand them harshly for their failure. At later meetings, these same officers propose drastically slashing the Joe team's operating budget to cripple their ability to engage Cobra forces. During a short battle at Joe headquarters, Duke is captured by the Dreadnoks and replaced with a synthoid. His imposter's inaction and seemingly inept leadership skills contribute to the Joe team's weakening state until they are ordered to disband.
The plan continues to work until Destro, vengeful after seeing the synthoid of himself, aides the Joes in securing much needed supplies from their depot, closed to them for weeks.
Synthoids are constructed from an unnamed form of protoplasm supplied by Destro. Acting more like fully compliant robots and answering only to Cobra Commander or Zartan, they do have a few vulnerabilities: they can be detected by dogs, like Mutt's Rottweiler Junkyard who barks angrily at the replica of Duke. Their structural integrity is controlled by a handheld device held by Cobra Commander. Whenever the Commander chooses, he may disintegrate a synthoid with this device, melting it into a pile of squirming protoplasm. Zartan warns that if the device is set on too high a signal, it will destroy all synthoids they've created.