The Thundercats are exhausted from fighting bad guys all the time so Lion-O suggests they spend a relaxing summer fun day at the beach, with no fighting. Unfortunately the beach is home to the Crabmen, giant, clawed humanoids that fight anyone who steps on their sand. Looks like summer fun day might involve some fighting.

The ThunderCats have gotten so good at defeating the Mutants of Plun-Darr that it's basically become routine. That all changes with the arrival of the Mutants' old leader: Ratar-O, wielder of the Sword of Plun-Darr. Uh-oh.