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In the conclusion of the four-part story "The Ark," the Doctor (William Hartnell) discovers that the space ark upon which the survivors of a plague on Planet Earth are being transported is now under the control of the Monoids, a formerly servile alien race. The only hope for restoring the balance of power is to force the Monoids to wipe out one another. Meanwhile, the Doctor's companion, Steven (Peter Purves), desperately conducts a search for a bomb that threatens to destory the Ark and all its passengers. Written by Paul Erickson and Lesley Scott, "The Bomb" originally aired on March 26, 1966.