Summoning Sylvia tells the tale of Larry, who has been kidnapped by his three best friends for a bachelor weekend getaway at a haunted house. As they sashay through the dusty corridors, the comrades recount the house’s legend from a hundred years ago: a murderous woman named Sylvia slaughtered her son and buried him beneath the house's floorboards. But then Larry remembers that he was supposed to spend the weekend bonding with Harrison, his mysterious future brother-in-law. Horrified at his oversight, Larry invites the army-uniformed, hetero Harrison to join his gaggle’s gaycation, neglecting to run it by his friends. Later, as they hold a high-spirited séance to summon the sinister Sylvia, the group ends up welcoming more houseguests than they bargained for.