The timeline of the Terminator franchise is ... tricky to explain. It has been rebooted, updated, there was a TV show at one point, timelines were created and they just keep sending Terminators back in time to mess with Sarah Connor.
As plans go, sending Arnold back every few years is a pretty messy one, and the trailer for Genisys begins to look awfully stupid when you stop and actually think about what's going on.
Which is why this fan edit is so good. If anyone in the film actually had to explain their thought process, including the fact that the guy John sends back must sleep with his mother for any of it to work, but of course they all exist so they already know it worked, but they have to do it anyway or bad things will happen ... it goes on and on.
There is still money to be had in the Terminator name, or at least the studio hopes there is. They'll keep mangling the central story to make sure there's always an excuse to send yet another Terminator back, but that doesn't make it any less stupid. This trailer does a wonderful job of skewering the central conceit of the movie, and it's definitely worth your time.