Sentinel Prime is the former Autobot leader and Optimus's mentor in Dark of the Moon (2011). He turned out to be one of the series' most unexpected villains.
A ship on the Moon
In the film, Sentinel's ship, the Ark, crashed on the Moon in the 1960s at the end of the war on Cybertron — the secret reason behind the space race. The Autobots find Sentinel in stasis and revive him with the Matrix.
Why he betrayed them
Sentinel believed the war couldn't be won and made a deal with Megatron: together they would bring ruined Cybertron into Earth's orbit and force humans to rebuild the planet. To him it was a way to save his people — at the cost of humanity's freedom.
Why he's interesting
Unlike many villains in the series, Sentinel has a clear motive and a tragedy: he's a leader who lost faith. He was voiced by Leonard Nimoy, who had voiced Galvatron in the 1986 film 25 years earlier.
The ending
Having survived his teacher's betrayal, Optimus destroys both Megatron and Sentinel in the finale. Many fans consider this storyline the film's strongest part.
