Selina (Carlson Young), the girlfriend of one of the victims, mentions a dump where they went to hunt Reinigen (rat-like Wesen) for fun, and informs them of a third missing Klaustreich, Johnny.
Monroe (Silas Weir Mitchell) and Rosalee (Bree Turner) explain the legend of the Riesen-ratte, "the rat king", a creature that is formed when several Reinigen with similar DNA combine into one being.
They manage to save Selina and stun the Riesen-ratte using power cables to shock the giant rat-king back into its individual parts.
Afterwards, Meisner (Damien Puckler), with Adalind's (Claire Coffee) help, manages to convince Nick to trust him, and they run to the hospital, where they save Trubel from a group of Wesen chanting "Occultatum Libera."
Meanwhile, Andrew Dixon (Michael Sheets) keeps trying to get Captain Renard (Sasha Roiz) to publicly endorse his run for mayor, promising him the role of Chief of Police.
The Reinigen apparently only become the Rat King when they are collectively terrified, which in this instance was almost certainly a symptom of the brewing Occultatem Libera uprising.
"[5] Liz Prugh from EW wrote, "Renard is still being recruited by Andrew Dixon and his campaign manager for an endorsement from the Portland PD.
"[6] MaryAnn Sleasman from TV.com, wrote, "Grimm's last two seasons have been phenomenal in terms of world-building and storytelling, giving us a lush and detailed world full of exciting, mysterious, and fascinating cultures and dangers.
Season 5 has struggled to recapture that, coming so close at times only to fail with Gotham levels of instability and K-Mart quality.