It happens sometime after Henry and Charles start dating and hiding the mer-secret from the others. One of their friends invites everyone to a bbq party on their new boat. (henry keeps his distance from the water.) Calvin notices something red swim by in the water, asking if anyone saw what looked like hair. He proceeds to believe he saw something and slowly thinks he's crazy when... (idk) they go fishing or find an abandoned net and Calvin relizes he's not crazy only to quickly relize "oh shit mermaids are real?" Ellie doesn't trust anyone going near her other than Henry which she calls "dolphin" as a nickname. So Cal being Cal, takes a slow and gentle aproach to gain her trust. The twins are sworn to the secret with Henry and Charles explaining things and Ellie begins trusting humans with Henry around.