“The story takes place in the year 2025, after Earth has welcomed alien beings to live peacefully with the human race. But peace is short lived, as the planet-conquering Troobian Empire turns its destructive attention to Earth. When the Earth’s first line of defense, the S.P.D. A-Squad, vanishes without trace, the protection of the planet falls to their replacements: the B-Squad Rangers, and their commander, Anubis Cruger.When two reformed thieves join the team as the Red and Yellow S.P.D. Rangers, tensions threaten to tear them apart. With the alien threat growing stronger every moment, the Rangers must put aside their differences and go into action as one!
Using teamwork, intergalactic weaponry and light-speed Zord vehicles to battle evil, they unite to become one of the ultimate forces for good: Power Rangers S. P. D.! It’s eventually revealed that the A-Squad is in fact evil and the B-Squad proves its might by defeating the former elite squad of SPD. With help from the S.P.D. commander and his wife Isinia, B-Squad destroys the true threat, Omni and Grumm is arrested for his crimes in front of S.P.D. headquaters on Earth by Cruger.”
-Taken from: http://powerrangers.wikia.com/wiki/Power_Rangers_SPD