Pursue the Undersea Base (E21)

The Ultra Guard launches a probe when a ship is dragged beneath the waves. Allegedly, the boat sent an SOS about a ship rising from the waves. It is identified as Ironrocks, a massive robot, and it begins destroying docks and dozens of vessels. The true master of Ironrocks, Mimy, appears and taunts Ultraseven before assaulting him and chaining him to Ironrocks, dragging him into the water. (Prime Video)