This is the theme for Doctor Who, the television series about a time traveller named the Doctor. He is a member of the race known as Time Lords, but disapproves of their generally hands off approach. Using his time machine, the TARDIS (Time And Relative Dimension In Space), he travels all around the universe and at different points in history, righting wrongs and fighting evil aliens, including enduring foes like the Daleks, the Cybermen, and even renegade Time Lords like the Master.
Time Lords — Time Lords are from the planet Gallifrey. They are able to travel through time thanks to energies harnassed by Time Lords Omega and Rassilon. Omega entered a sun going nova and found a way to send the energy back to Gallifrey. Rassilon's task was to enter a black hole and locate the Eye of Harmony, which enabled the Time Lords to essentially exist outside of time. They entered a golden age and gained the ability to 'die' and regenerate into new bodies twelve times. Early efforts in altering time lead them to mostly ban the practice.
The Daleks — Perhaps the Doctor's most enduring foes, the Daleks are humanoid lifeforms mutated so badly by radiation that they are little more than tentacled blobs. They are encased in robotic armor to give them mobility and weaponry. Originally created by mad scientist Davros, they turned on their creator and set out to rule the universe. With no compassion or morals to worry about, the Daleks are only concerned with being the supreme beings in the universe. All other lifeforms are to be exterminated or subjugated if they can be of use, only to be killed with they are no longer useful. The Daleks have acheived space travel as well as time travel. They are cold, calculating, and the only emotions they are capable of feeling are anger and hatred. Their main weaknesses are the inflexibility of their minds and armor, but thanks to hover systems, they are no longer stymied by stairs.