This is an excellent time travel episode, and also one of the best Harry Kim episode in which his future him is much tougher than the Harry Kim we know.
When Voyager crashes after a new travel attempt with the slipstream drive, the whole crew dies - almost. Harry and Kim, who were flying in front of the ship in the Delta Flyer, survived and lived their lives for 15 years afterwards. Both spent most of the past years finding a way to undo this event. Especially Harry's life only was about redeeming himself and bring the crew back, because it was his wrong calculation which led to the Voyager crash. After they obtain a temporal device from the Borg, stole the delta flyer and found the crashed Voyager, they try to send a message to past Seven of Nine and provide her with the correct calculation to correct the mistake done by Harry. Although this attempt fails, too, they still manage to save Voyager and the crew and revert the past 15 years with a snap of their fingers.
I like time travel stories and this is a well done episode. Plagued by his own mistake, Harry is obsessed with making things right and bring the crew back. His past 15 years must have been painful for him. Well acted by Garrett Wang in this episode. Normally I am not a fan of this Harry Kim character, but this episode was joyful to watch.
But just like with every time travel episode, they always pick a few minutes before the fatal event to change the timeline. Since they are in the future, they could have chosen any time to send Seven a message through her Borg implants. For example when they were debating Harry's suggestion to execute the flight. With an additional message that explains everything, Harry could have stopped the crew from trying this flight in the first place. This way it would not have been such a pressure to fix everything in the last possible second. But of course, this would be boring to watch for the viewers. Also, Chakotay gets the shortest straw in this episode. Although he also determined his life in changing the past and even sacrifices the new love he found back on earth, it is only Harry who receives all the praise. This is one of the letdowns in this episode. Chakotay is the unsung hero of this episode.
I wonder though: Where did the doctor get his mobile holo emitter back? Last time it was evolved by Borg technology and fused into the Brog drone. Suddenly it is up and running again. And La Forge was quite pointless and helpless in this episode. He could not even stop a shuttle craft with his spaceship.