No Way Back (S04E26)
The Seaview suffers a catastrophic power surge, resulting in an explosion that claims the lives of its entire crew. Meanwhile, Admiral Nelson, who was at the Nelson Institute during the tragedy, receives an intriguing proposition from Mr. Pem, the survivor from "A Time to Die." Pem offers Nelson the chance to use the submarine's nuclear reactor in exchange for transporting him back in time to prevent the explosion. Once there, Pem activates his time device, sending the Seaview even further back to the American Civil War. Before the crew can grasp the situation, they find themselves confronted by Major General Benedict Arnold. Nelson must thwart Pem's ambitious scheme for world domination while also trying to save the Seaview from destruction.