Just before the first tsunami wave hitting, a young girl is flying a drone on the pier and a couple is getting married on a boat nearby. 

The first wave hits and Buck struggles underwater. When he can get above water, he finds Christopher and the two seek safety on top of a fire truck. 

A flashback to shortly before the first wave hits, shows Athena and May getting in a car accident. 

Overrun with calls, 9-1-1 is forced to put callers on hold. After losing a man on the line who drowns in his attic, Maddie is tasked with checking in on the 9-1-1 operators and helping coordinate care for those injured. 

The 118 sets out to help victims, and they encounter the couple who were on the boat getting married. The groom and his future stepson are pinned together by a radio antenna that is connected to the boat. After extracting the son, Bobby has to dive under the boat the free the antenna so the groom can be brought to safety. 

May helps a woman who is severely injured after the pile-up. 

As another wave hits, the fire truck is overrun and sends people flying into the water, including Christopher. Buck screams for him but is unable to locate him.