From the Designer's Website:
Jouez un cétacé, protégeant les hommes des démons de l'abysse...
User Translation from French:
Play a cetacean, protecting mankind from the demons of the abyss...
("L'autre côté du miroir" is French for "The other side of the mirror".)
Characters have six attributes: nage (swimming), manoeuvre (manoeuvrability), dégâts (damage), résistance (toughness), sens (senses), intelligence (intelligence). These are rated from 1 to 4, with 15 points available to spend on them at character creation. Attributes with a rating of 1 have an associated weakness. Attributes with a rating of 3 have a speciality, and of 4 have two specialities. An additional speciality may be bought by also taking a corresponding weakness in the same attribute.
Task resolution is performed by rolling a number of six-sided dice equal to the appropriate attribute value, keeping the single best die roll. This may be adjusted by one point in either direction if the character has a particular weakness or speciality in that area. If the result is equal to or higher than a target difficulty number (or an opponent's result if in an opposed action) then the character is successful.