Elisa Esposito is a mousy, but intelligent and perceptive woman who was rendered mute by an injury she sustained as an infant. She works the night shift as a janitor at the secretive Occam Aerospace Research Center in Baltimore during the Cold War in the early 1960s. One day, the facility receives a new “Asset” which has been captured by the abusive and cruel Colonel Richard Strickland from a river in South America.
After Elisa has a brief encounter with the Asset, which she discovers is an amphibian-humanoid creature, she begins sneaking into the room where it is restrained by heavy metal shackles and chains. Elisa secretly spends time with the Asset, feeding him hard-boiled eggs and sharing Big Band music by Benny Goodman and other musicians through records played on a portable phonograph turntable. Over time, the two begin to trust each other and become friends, even though neither can speak.
Strickland’s superior, General Hoyt, is interested in the Asset’s anatomy and wants to dissect or vivisect its body for research to advance space travel technology. One of the center’s scientists, Robert Hoffstetler (who is actually a Soviet spy named Dmitri), pleads with Hoyt and Strickland to keep the creature alive for further study. Robert’s plea is dismissed, but Elisa has overheard the entire conversation. In addition, Robert’s secret spymasters instruct him to euthanize the creature, preferring to prevent American researchers from gaining an advantage over Soviet scientists.
Desperate and afraid, Elisa convinces her next-door neighbor, closeted commercial artist Giles to help break the Asset out on the day it has been scheduled for dissection. Meanwhile as Robert wonders what to do, he discovers Elisa’s plot to escape with the Asset, and decides to help them. When Elisa encounters her talkative friend and co-worker Zelda, she is forced to reveal the Asset’s existence as she and the creature escape with Robert’s assistance.
Elisa submerges the creature in her apartment bathtub, keeping him healthy and alive using special water conditioning chemicals smuggled out by Robert. The next night, Strickland interrogates both Elisa and Zelda, but quickly turns his suspicions elsewhere due to their menial jobs.
As Giles keeps watch over the creature living in the bathtub, he dozes off. The creature then kills and partially eats one of Giles’s cats, but alarmed at being discovered, runs off after slashing Giles’s arm with its claws. After Elisa finds, calms, and returns the Asset to her apartment, she and Giles both discover it can become luminescent when touched. The next morning, the once-balding Giles discovers his hair growing back and the slash on his arm completely healed. Elisa and the Asset become intimate, and make love for the first time.
Knowing the Asset cannot remain indefinitely in her apartment, Elisa reluctantly plans to release it within days into a nearby canal; there will be few witnesses and it can swim over a barrier when rainwaters flood the canal. Meanwhile, Hoyt threatens Strickland’s job if he does not recover the missing Asset within 36 hours. At the same time, Robert’s spymasters become suspicious that he did not kill the Asset as promised and may have hidden him instead. Ominously, they tell him that he will be “extracted” in two days’ time.
No longer able to provide the artificial water conditioning chemicals the Asset needs to survive, Elisa and Giles both have seen its health deteriorating, forcing them to act. The time of the Asset’s release approaches as heavy rain arrives. Strickland secretly follows Robert to his meeting with his two Soviet spymasters. They fatally shoot him, but before they can finish the job, Strickland kills them. His Soviet connections revealed, Robert is tortured by Strickland into giving information about who rescued the Asset. Robert soon implicates Elisa and Zelda, and is abandoned to die of his wounds. Strickland then forces his way into Zelda’s home and threatens her while seeking more information, causing her terrified husband to reveal Elisa’s connections to the escape. Searching through Elisa’s vacant apartment, Strickland finds a note on her calendar revealing her likely location.
At the edge of the canal, Elisa and Giles give sad farewells to the Asset, but Strickland suddenly arrives. He punches Giles and shoots the Asset and Elisa to stop them from escaping. Giles manages to get the upper hand on Strickland while the Asset instantly heals his own gunshot wounds before slashing Strickland’s throat with its claws. As Zelda and the police arrive at the scene, the Asset pulls Elisa into the canal, where he uses his abilities to heal her wounds. The creature transforms the scars on the side of her neck into amphibian gills, giving her the ability to breathe underwater and allowing them to escape together.
Sally HawkinsElisa Esposito, a mute janitor at a secret research facility
Michael ShannonColonel Richard Strickland, the man in charge of the creature research team
Richard JenkinsGiles, Elisa's friend and neighbor, a closeted commercial artist
Octavia SpencerZelda Fuller, Elisa's friend and co-worker
Doug JonesThe Asset, an amphibian-humanoid creature, held captive at the facility
Michael StuhlbargDr. Robert Hoffstetler, one of the lab's scientists, working secretly as a Soviet spy
Lauren Lee SmithElaine Strickland, Richard's wife, who raises their two children in a stereotypical suburban home
Nick SearcyGeneral Hoyt, Strickland's gruff and demanding sponsor