Challenger sends telegrams asking his three companions from The Lost World - Edward Malone, Lord John Roxton, and Professor Summerlee - to join him at his home outside of London. The cryptic telegrams also instruct each of them to 'bring oxygen'. On arrival they are ushered into a sealed room, along with Challenger and his wife. In the course of his researches into various phenomena, Challenger has predicted that the Earth is moving into a belt of poisonous ether which, based on its effect on the people of Sumatra earlier in the day, will cause the end of humanity. Challenger seals them in the room with cylinders of oxygen, which he (correctly) believes will counter the effect of the ether. The sealing is not to keep the ether out - it permeates everything - but "to keep the oxygen in".