When a mysterious rift appears over Manhattan, Rex is on the scene to see if there is trouble coming. However when Humungosaur comes from the clouds, Rex gets a bit more than he bargained for! Rex begins battling it out with the Vaxasaurian in Central Park, not realising that Humungosaur is actually Ben Tennyson. Ben soon discovers he's in an alternate dimension and that he and Rex must team up to overcome their combined foes.