Ultra Q (1966) : 1x6

A young boy named Taro develops a curious preoccupation with his pet turtle named Gameron, to whom he constantly encourages to grow. After getting into trouble for bringing his turtle to school and neglecting his studies, Taro winds up in the company of two bumbling gangsters as they flee from the authorities. While hiding out in the sewer, Taro is delighted to find that his turtle has suddenly grown to 99 cm in length. The sight of the giant turtle, however, frightens the gangsters into the welcoming arms of the police. Assuming that Taro had been kidnapped by the two thieves, the authorities return to collect the boy, who is nowhere to be found.