The episode begins in the armory with "Trip" in a photon torpedo casing and the senior staff gathered around as Archer gives a touching eulogy about sacrifices.

After the jazzy remix of Faith of the Heart plays we get the obligatory "Two Weeks Earlier" message so we can find out what happened.  We start with a painful as always Neuropressure session between Trip and T’Pol where they are rubbing each other’s feet.

Trip tells T’Pol that he has a plan on how he can get more efficiency out of the warp engines, but when implementing his plan something goes wrong and the ship nearly explodes, Trip saves the ship but at a cost, he flies across the room and bashes his head on the ground.  He goes into a coma.

Never fear, Phlox is here!  The good Doctor happens to have an alien in his stash that secretes a virus-suppressing agent, but Trip doesn’t have a virus so what’s the big deal about this alien?  Well, it turns out that this alien has another interesting feature when injected with DNA from any species, this alien will immediately become an exact duplicate of that creature and start living out a vastly accelerated version of that creature’s lifespan.  The creature will go from embryo to baby to child to teenager to adult to old to death in a matter of just 15 days.  Why is this important?  Because Trip needs a brain tissue transplant and nobody on the crew is a match.

So after agonizing about it for a while and ignoring T’Pol’s arguments that cloning for the purpose of doing a tissue transplant is unethical, Archer agrees to allow Phlox to do the procedure.

In order to do a rapidly aging clone properly you would probably have to make it so that though the body ages quickly the clone still needs to learn like a normal human would have to which would mean that 8 days in when the clone Trip was of age for the transplant, he was still acting like a baby because in actuality he is only 8 days old, well if the writers did that we wouldn’t grow emotionally attached to the clone, which is necessary to feel bad when Sim (the name Phlox gave to the clone) dies.  So instead the writers gave Sim an interesting, if ridiculous, ability.  As Sim ages, he slowly gains the memories of Trip, for example when Sim is the equivalent of an 8-year-old, he has the memories Trip did when he was 8.

So we have scenes where Phlox and Hoshi are reading with the "4-year-old" Sim and Sim asks if they could jump ahead in the story to his favorite part.  When Hoshi asks him how he knows of this part they haven’t gotten to yet, Sim answers that his Mom read the story to him.

Then we have the "8-year-old" Sim hanging out with Captain Archer.  Archer lets Sim play with his model spacecraft that we first saw a young archer playing with way back in "Broken Bow."  Sim asks where his parents are and Archer takes him to sickbay and shows him the original Trip and explains to Sim that Sim is a copy.

As Sim gets older "17-year-old Sim" starts helping out in Engineering and has a crush on T’Pol.  When Sim reaches adulthood and starts being played by Trinner he comes up with a plan to tow Enterprise out of the field of weird magnetic particles that are engulfing the ship.  Oh, I haven’t mentioned those have I?

The experiment at the beginning of the episode ended with Enterprise screeching to a halt in this weird field of strange unidentified magnetic particles.  These particles keep clinging to the hull, which in and of itself is not that bad but as more and more of them keep showing up, more and more stress is taking place on the hull, eventually if Enterprise can’t get out of this field they will be crushed.  Sim has this plan to use both shuttlepods to tow Enterprise out of the area.

Sim begs Archer to be allowed to fly one of the shuttles claiming "It’s my plan I should be the one out there," and when Archer denies him he answers back "You are just worried about something happening to me so you can’t save Trip aren’t you?"  Archer actually answered back that that was true.  The plan does work and Enterprise is saved.  In fact, as soon as Enterprise exits the field all of those weird particles leave the hull and return to the field.

Meanwhile, Sim has gotten all caught up in Trip’s memories and there is a scene between Sim and T’Pol in which Sim says something like "I don’t know if these are his feelings or mine, but I’m in love with you."  T’Pol dismisses it in typical Vulcan fashion and we continue.

Now we get to the drama that we’ve all been waiting for, it has come time for the operation, but oh no, what’s this, Phlox has discovered that Sim will not survive the operation.  What’s more, Sim brings up another moral dilemma when he points out that there is an experimental enzyme that potentially has the ability to slow his rapid aging, theoretically allowing him to live out the rest of a normal life.

Archer is torn but the enzyme’s effectiveness is unknown so he tells Sim that saving Trip is the only way either will conceivably survive.  Sim goes and asks T’Pol if he can spend his last few hours in Engineering as the two things he cares for the most are in that room, but instead seizes control of the shuttlebay and plans to steal a shuttle to escape.  When Archer arrives in the shuttlebay however Sim is still there, claiming that he didn’t want to spend the rest of his life floating in space in a shuttle pod, and how if Trip is that important to Enterprise saving Earth, he doesn’t want anything to happen to anyone else the way it happened to his sister, whom he claims is just as much his sister as she is Trip’s.

So Sim heads over to sickbay to die but first says goodbye to T’Pol who gives him a farewell kiss.  Sim goes to sickbay, and tells the unconscious Trip "You owe me one." And undergoes the procedure.  The episode ends the way it began, with Sim’s funeral and Sim being launched out into space.