Following the success of the first Spin & Marty serial on The Mickey Mouse Club, Disney secured a license with Grosset & Dunlap & the Strathmeyer Syndicate to adapt The Hardy Boys into a serial. Two were produced, one each in seasons 2 & 3.
Tim Considine (Spin & Marty) & Tommy Kirk starred as juvenile detectives Frank & Joe Hardy in what would be the first of five TV incarnations of the long running franchise. Disney de-aged the sleuths slightly to pre-teens to keep in line with the pre-teen Mouseketeer roster, I would guess.
The first serial, "The Mystery of Applegate's Treasure", was adapted from the first book in the series, The Tower Treasure. Jackson Gillis, later a producer on Perry Mason, wrote the script, and followed with a completely new tale the next season. Here's the open:
Tim Considine would later join the cast of My Three Sons, and Tommy Kirk went on to make a few movies for Disney, including the original "Shaggy Dog".
No rating. Never saw it, even during the rerun period in the 70's.