All this week, BBC America is building up to Saturday’s global simulcast of the 50^th anniversary special episode of Doctor Who. The week’s festivities begin early today. At 8 a.m. ET, BBC America repeats Doctor Who: The Companions, its special devoted to the Doctor’s many traveling companions through the (past 49) years. Then, beginning at 9 a.m. ET, the network presents a 10-installment marathon of The Doctors Revisited, with individual one-hour specials devoted to the first 10 incarnations of Doctor Who, including Jon Pertwee, shown here. (The 11^th and current Doctor, played by Matt Smith, gets his own special later this week.) Consider this all-day marathon the perfect way to get up to speed on one of the fantasy genre’s most iconic series – a virtual Doctor Who’s Who.