Flash Gordon: Deadline by Noon (1955) [colourised]

Starring Steve Holland as Flash Gordon from the 1955 series.
An enemy travels back in time 1250 years to plant a bomb in 1953 Berlin. Flash, Dale, and Dr. Zarkov have less than and hour to find the bomb in their time and diffuse it or the earth will be destroyed.

This episode is public domain:
https://archive.org/details/FlashGordon-DeadlineAtNoon-1955