The Last Mineral (III) - S1.E23

Whilst she is dining with him at Roberto's Restaurant, the others reclaim the minerals, notes and Salamanders from Sator's strong hold. But Sator returns before they have finished and captures Peter Poopsnagle. This is when Peter is cruelly reunited with his grandfather. Undaunted, the doctor and children go to the Opera House, where they are successful in discovering that the final mineral is water. Back at the doctor's laboratory, they are working on the Mega Steam problem when Sator contacts them on Matt's communicator. He tells the doctor that unless he gets the notes back by the next morning, they will never see Peter or his grandfather again.