The Trap of the Super Spacemen - S3.E5

When a returning astronaut turns out to be a powerful alien, Superboy and Krypto head off into space to investigate. They discover that the real astronaut has been kidnapped. Superboy is attacked by the aliens and imprisoned. The aliens plan to use him as a power source for an invasion of Earth. Krypto manages to rescue Superboy and the two of them put a stop to the aliens' plans.