Its one hell of a complicated film. It will be very hard for an average viewer to gather all the information provided by this movie at the first watch. But the more you watch it, more hidden elements will come to light. And when you are able to put these hidden elements together. You will realize that this movie is just a “masterpiece” which takes the legacy of Christopher Nolan Forward.
Here I will do my best and explain the timelines of the 4 major characters. All characters go through a RED timeline and a BLUE timeline. RED is moving forward and BLUE is moving backwards in the RED timeline. I hope that makes sense. Here we go!

1) The Sator Timeline. When he’s a young man he finds a box while working in Russia. The box contains gold and the BLUE technology. He’s on a mission to find pieces for a doomsday devise that wipes out existence. The last piece he needs is missing. He threatens Kat’s life with the Protagonist.
He uses the Blue technology to recover the missing piece. He now has all the pieces for the doomsday device. He than goes back to the day on the yacht in the RED timeline. The day where Kat sees SOMEONE dive off the boat and says he disappeared.

2) TheProtagonist timeline. After the opening opera scene he is tortured, passes a test, and is recruited. He learns of the BLUE technology. He gathers information to lead him down the path and finds the missing piece for Sator. After Kat gets shot he then uses BLUE technology so she can heal. She can heal in the BLUE timeline because she was shot with BLUE technology. Only way to come back to the RED timeline was through another BLUE and RED machine. Only one he has seen was at the airport.
So the Protagonist must get them back to the RED timeline using the airport BLUE and RED machine. Protagonist then goes back to the day of the yacht in the RED timeline. Where they remove the doomsday device before the explosion.

3) The Neil Timeline. Neil helps the Protagonist in the now RED timeline. Not Neil’s RED timeline but the Protagonist’s RED. They find what they need to find. After Kat gets shot he goes into the BLUE timeline and along to the airport to get her through the RED and BLUE machine. He then travels to the day of the yacht in the BLUE timeline. As he is in the cave he sees another BLUE and RED machine. Knowing that they caved in the tunnel with explosives he needs to be at multiple places at once.
So he then travels again through the BLUE timeline to intercept the Protagonist’s failure so the Protagonist does not fail. He gets killed in the process. Keep in mind that when he went through the BLUE twice one of his selfs had to go to the RED timeline. This is where he meets up with them at the end. This is where the Protagonist realized he’s dead and it was him who saved him at the opera house in the beginning.

4) The Kat timeline. When she meets the Protagonist in the movie it’s in the RED timeline. She tells a story about the yacht and she see a woman dive off the boat, the woman diver is herself. Later, Kat gets held hostage by Sator in the RED timeline. Sator is in the same timeline but in BLUE. After the Protagonist saves her they are both captured. Protagonist captured in RED and Kat is captured in BLUE. After she gets shot she heals (Airport scene). She then heads to the day on the yacht in the RED timeline. She sees her past self leave the boat with her son and sees Sator leave in his helicopter. Ok this where it gets tricky… Sator comes back but it’s not the RED Sator it’s the BLUE Sator from the highway scene after he got all the pieces from the doomsday device.
The RED Sator left and the BLUE came back in the RED timeline. She kills him. She always killed him. She said earlier at dinner with the Protagonist that she saw a woman dive off the boat and her husband had disappeared. She killed him and slid him off the boat with the lotion she put on his back.

Whatever happened was always going to and did happen. Neil says that a few times. But it wasn’t until the end where he says it doesn’t mean you should sit around and do nothing about it. Kat always kills Sator that day, Neil always dies that day.I hope that clears up some confusions. I did my best without going into the smaller details. Please react if this was helpful or not.
Thank you reading those that made it until the end.
For More Vist Broadway Originals