An Otter Emergency

Harry ends up in St. Mungo's with an otter hanging off his arm, clinging to him by its teeth. By the way, Hermione is an Otter Animagus. Even more unfortunately for Harry, not all is as it seems. TL; DR: Harry's abysmally bad luck and general obliviousness bite him in the -behind- arm. Literally. Cross-posted to AO3.