Did you know that a group of snails is called a 'flock". Well it is in Julia Donaldson rhyming world and one of a group who live rather mundanely on an harbour rock dreams of going out to sea and exploring the world. Using it's very own snail-trail, it writes a message that attracts an enormous humpback whale who invites this tiny little gastropod to slide onto his tail and off they go. Luckily, he is quite adhesive so doesn't get washed off as his giant host takes him on an enjoyable trip for both us and the animators who get to present us with a submersible travelogue that shows the delights and dangers of the deep, has some fun with some hungry sharks and even manages to involve some human kids and some human kindness. This is a charmingly simple story that sort of encourages the grass is greener philosophy, but equally extols virtues of inter-reliance, friendship and courage. Maybe best not look at the whale snail scale too closely, but that's just a detail for this finely drawn film.