The Doctor experiments on the seaweed, which when returned to water grows in size and strength, and seems to become malevolent before they successfully seal it in its aquarium. Meanwhile, Robson and Harris continue to antagonize one another, Robson becoming more and more strident and emotional. Exhausted, he retires to his room to rest, and is locked inside by Mr. Oak. As the heartbeat noise fills the cabin, sea foam gushes from the air vent, and Robson wakes to see a creature behind the vent trying to get through. The Doctor, already on his way to see Robson, is alarmed by Robson's screams and forces the door. Robson flees in terror and the Doctor and Harris get a glimpse of the creature before it goes back inside the vent.