Hydromander

Ben, Gwen, and Max arrive at Niagra Falls, where Ben is warned not to go into the water, as he can be a little bit 'reckless' in it.

Later than night, Ben sneaks out of the Rustbucket as Cannonbolt and does a cannonball into the water, splashing a nearby fisherman and knocking a rock out of place, waking up a Hydromander. This Hydromander starts 'feeding' by eating everything around it.

The next morning, this wakes Ben, Gwen and Max up, so Ben uses a new alien, Overflow, to fight it, ultimately being defeated. He goes back to the Rustbucket to complain, but meanwhile, the Hydromander finishes its feeding stage.

Ben and co. find it laying down, presumably sick, in the water. Ben does a victory dance at this, before its skin bubbles up, and it sinks beneath the waves only to be replaced by more, smaller Hydromanders. Ben transforms into Heatblast to fight them, but he is overpowered by them when they multiply faster than Heatblast can hit them.

The Hydromanders are about to crush Ben with a rock, when he dodges and turns into Overflow and starts attacking again. In this time, Gwen discovers that they are weak on land, so she tells Ben to 'do what he always does' and Ben does a huge cannonball into the water, knocking out the Hydromanders.

Later, the Hydromanders are locked up and Gwen apologises for doubting Ben, who turns into Cannonbolt and does another cannonball into the water, soaking Gwen and Max.