in the bloodstream, rachel notices a pathogen while looking at all of the different types of blood cells around them. they realize that when the helmacrons reach the heart, they can stop it from beating with their weapons. marco goes back to the apartment a second time, and while he is there someone comes home. he soon finds himself trapped in the closet, and against jake's orders, morphs a cockroach. the helmacrons shoot at the cockroach's heart, after which everyone believes marco is dead, until they remember that cockroaches can withstand severe trauma. in the meantime, the animorphs get the weapons away from the helmacrons by biting their legs. marco is indeed alive, and they manage to get him to morph back to human. holding the helmacrons at gunpoint, they all return to the barn and recharge the helmacron's ship with the morphing cube. the helmacrons leave earth again after promising to never come back.