S1·E118 Kidnapped Gash! Trap Over the Sea. Sinking Dartagnan.

A professor named Dartangan visits Kiyo and tells him he has taken Kiyo's research experiment before jumping out the window. Megumi and Sunbeam arrive and ask Kiyo if he has seen Tia or Ponygon. They decide to investigate the park and find a message left by Dartangan that tells Kiyo if he wants to see the children again, he must go to the pier and board Dartangan's boat.