TERRY Jones and Alan Rickman are here united by fish. Well, the chummy eccentricity of Jones supplies the voice of a loopy professor who believes that, since the world will soon be all water, we just have to become fish, so he has naturally devised a magic potion.

The fruity, villainous tones of Rickman breathe life into the ugly fascist fish who, having discovered that the potion's antidote causes fish to think like humans and speak fluent English, decides he wants the world for himself.

He is particularly hostile to the three jolly children who join him, having become fish versions of themselves in the process: impulsive, optimistic Fly, his cute, giggling sister Stella and their plump, intellectual cousin Chuck. Their parents, meanwhile, fret over them in the world above water.

In the wake of Shrek (where the soar-away technology was matched by invention in all other areas), you might think children would find any cartoon failing hugely by comparison. But the simple, flattish, traditional animation here (which is also dwarfed by Disney) is at least beautifully coloured, while the fluent underwater movements and wash of detail (especially in the undersea city) always engage. So does the story, which is buoyed by charm, innocence, eccentricity and a nippy pace.

The conventional moral themes of the cartoon are there in full measure (the responsibility of science; being nice to one another), but what you will take away with you is the memory of the dark, scheming fish demanding servants and fame, the jellyfish with specs and the military crab with the lantern jaw and scary teeth.