Contains spoilers for this episode and earlier episodes.
The town escapes a heat wave at the public pool, where Dipper takes an assistant lifeguard job to spend time with Wendy. Her relaxed approach to work clashes with the strict rules of their boss, Mr. Poolcheck, but Dipper enjoys having her attention. Mabel becomes interested in Mermando, a boy who never seems to leave the water. He reveals that he is a merman, separated from his family after being caught at sea and transported inland. Mabel wants a romance, yet seeing how much he misses home persuades her to help him leave. Stan has a separate battle with Gideon over the pool's best deck chair.
Mabel returns after closing with a cooler to carry Mermando to the lake. Poolcheck has ordered Dipper to guard the missing supplies, so he intercepts his sister and tries to stop her departure. The chase ends beside the lake, where he learns whom she is actually rescuing. When Mermando struggles to breathe, Dipper supplies water through an improvised rescue. Mabel needs a megaphone to help her friend call his family, and Dipper finally agrees to risk his job for them. Mermando kisses Mabel before swimming away, promising to write. Both Dipper and Wendy lose their pool jobs, but Wendy is relieved to be free of Poolcheck. Mabel keeps the memory of her kiss and her new friend's letters.