So since Power Rangers is apparently going to be a six-movie storyline:
1) The origin story
2) Rangers vs. Tommy Oliver
3) End of first trilogy: Rita Repulsa is defeated for good. Jason, Zack, and Trini leave and in their place, we get Rocky, Adam, and Aisha
4) Tommy returns as the White Ranger, and Lord Zedd takes over as the Big Bad. Also, Kat Hillard arrives and the start of the Kimberly-Tommy-Kat love triangle.
5) Alien Rangers
6) End of second trilogy: Lord Zedd is defeated. Mighty Morphin era comes to an end, start of the Zeo era