S4·E26 No Way Back

Seaview experiences a sudden power overload and blows up, killing all hands aboard. When Admiral Nelson (who was at the Nelson Institute at the time of the accident) hears of the destruction of the Seaview, he gets an impossible to refuse offer from Mr. Pem (who survived from A Time To Die): in exchange for the use of the sub's nuclear reactor, Pem will transport Nelson back in time to before the explosion which destroyed Seaview. Once back in time, Pem renergizes his time travel device and brings the sub back even further in time to the American Civil War. Before the crew has time to absorb this, the ship is boarded by Major General Benedict Arnold! Now Nelson has to stop Pem's far-reaching plan to take over the world and avoid the explosive disentergration of Seaview.