This was definitely one of the best episodes of the first season. It showed you how the Heroes where five years in the future.
The biggest change was definitely Hiro. The happy go lucky guy was gone, and instead there was a very sad person trying his best to return to the past so that he can stop the explosion from happening. We get to know that during the explosion Ando died and that is why he wants to go back.
Peter Petrelli became a very different person. The person that always wanted to help people had died in the explosion. Instead there was an indifferent person who cared about nothing. He was dating Nicky who worked as a stripper by the name of her alter-ego Jessica. Nathan Petrelli became the President of the United States. He set one thing in motion that of the mass murder of all the Heroes. Mohinder Suresh who was his assistant definitely was against what the President wanted to do.
Things started to change when the present day Hiro and Ando went to the future. Peter realised that it was a good chance to change the past. He revealed to Nicky that it was him not Sylar that caused the explosion and it was all his fault that DL and Micah died. He went to help Hiro and Ando.
Mohinder also helped Hiro when he was going to be killed under the orders of the President, because even he saw that if there was a slight chance of stopping the explosion they had to take it.
When the President knew that Peter was helping Hiro he flew over there, and it was only then that it was shown that it wasn't Nathan but it was Sylar. He had killed Nathan, took his powers and changed his look to look like Nathan. He was pretty much invincible and had the top position in the world. A final battle between Peter and Sylar was taking place, whilst Hiro and Ando were returning back to present time.