When the title is the funniest thing on offer, you have a problem if you're a cartoon. This Danish full-length animated feature starts well, loses its way in the middle and struggles to finish in one piece.

Voiced in English and American, it is aimed at an international child audience. The concept is not without charm and the characters, particularly Chucky, the fat brainy cousin, who comes over to Fly and Stella's place, because his mother is babysitting, are great.

Fly is a typically boyish boy, skateboard under the arm, baseball cap back-to-front, no interest in books or TV. He likes doing stuff, like fishing. He forces Chucky, who hates water, the outdoors and sport, to come along. When Chucky's mum falls asleep, reading a story, little Stella joins them. And then the tide comes in, trapping the kids on a rock, inside of which, through a secret door, floats the underground laboratory of Professor MacKrill (Terry Jones on vocals), who has invented a potion that changes people into fish.

When no one's looking, Stella takes a swig and turns into a starfish. Chucky throws her into the sea, without knowing it's Stella, and then Fly and he have to take the potion to try and find her. So far so good.

The undersea adventures are dominated by two new characters, a shark and a pilot fish, that are nothing like as interesting as the others. The joke is that the pilot fish (Alan Rickman) takes a sniff of the Prof's antidote, which feeds his brain enough to tell the shark, "No more cleaning," and turn him into a fascist dictator.

Will the children escape the murderous shark, now working as the pilot fish's killing machine, drink the antidote and become human again before their time runs out? The script cheats somewhat, the pilotfish/shark subplot loses its bite and the writers' imagination suffers from rising damp.

Chucky as a jellyfish almost makes up for it, but not quite.