No Way Out

So there's these dudes called the humans, and they are going along doing what humans do, which is fight wars between themselves and say "We should totally build spaceships!" Then one day, some guy actually built a spaceship. Because spaceships were in that year, everybody started building them. So soon the humans had united under one banner and were happily colonising worlds, when an alien threat appeared. Aliens who traveled the galaxy in eel-like bio-ships were looking for a new planet to call home after their old one was destroyed when its star went nova appeared on earth and started generally destroying stuff. The entire human fleet was wiped out easily. One ship remained. It was loaded with the most powerful superweapon known to man, an antimatter bomb, and sent down the throat of the bio-ship. Only if it can reach the heart of the beast will the bomb work, and only then will the ship, the eel, and the aliens be eliminated. And of course you get the fun job of carrying the hopes of all of humanity. So don't screw it up.