Waterfilter is the first episode of Ben 10.
Ben, Gwen, and Max arrive at Niagara Falls, where Ben is warned not to go into the water, as he can be a little bit 'reckless' in it. Gwen offers to be his 'filter' to tell him when one of his ideas is really terrible. Ben, however, wants to go by his "Ben-tuition" as he believes it can never go wrong. But on Grandpa Max's advice, he reluctantly agrees.
Later that 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, the din wakes Ben, Gwen and Max up, who see the rampage caused by the Hydromander. Ben decides to fight it using a new alien, Overflow, against Gwen's warning, and is ultimately being defeated. He goes back to the Rustbucket to complain, but meanwhile, the Hydromander finishes its feeding stage.
Ben and Gwen 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, on Gwen's advice, transforms into Heatblast to fight them, but he is overpowered by them when they multiply faster than Heatblast can hit them. They end up throwing him into the water, where his powers are useless. Gwen admits her mistake and asks Ben to use his "Ben-tuition".
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 from water onto land.
Later, the Hydromanders are locked up and Gwen apologizes for doubting Ben, who turns into Cannonbolt and does another cannonball into the water, soaking Gwen and Max, much to their disappointment.
Omnitrix Alien Debuts
- Ben Tennyson (first appearance)
- Gwen Tennyson (first appearance)
- Max Tennyson (first appearance)
- Todd (first appearance)
- Sally (first appearance)
- Hydromanders (first appearance)
- It aired as the second episode in the U.S.
- It was added to the Cartoon Network website as a sneak peek on February 22, 2017.