First of all, Rainsford has nowhere else to go after falling off his yacht and swimming ashore on the mysterious Ship-Trap Island. Although his travelling companions may be counted upon to search for him, Rainsford must realize that it may be days or weeks before he is rescued. The isolated locale of the island precludes any outside interference, so Rainsford knows he is on his own. Zaroff knows every inch of his island, and he gives Rainsford just enough information about the dangers there to keep his "guest" on his toes. Rainsford decides that he has a better chance of eluding Zaroff with the rocks below; and in the end, when Rainsford confronts the surprised Russian in his bedroom, Rainsford also realizes that Zaroff will have no one to save him when he continues the illicit hunt.