This episode's headline: Mission Impossible?; The Day After; Conversation. The guests include SEN. PAUL SIMON, [D] Illinois; SEN. JOHN McCAIN, [R] Arizona; REP. ROBERT TORRICELLI, [D] New Jersey; SEN. RICHARD LUGAR, [R] Indiana; YEVGENY YEVTUSHENKO, Russian Poet; CORRESPONDENTS: IAN WILLIAMS; NIK GOWING. Byline: In New York: ROBERT MacNeil; In Washington: JAMES LEHRER