Nanna Ingvarsson and Davis Hasty in One Flea Spare.

In One Flea Spare, Naomi Wallace’s bubonic plague-set drama, director Alexander Strain moodily captures the claustrophobic feeling that results when four potentially infected individuals are quarantined in a room together. But when the constrained mood of Forum Theatre’s latest production is combined with the work’s sluggish pacing and unsurprising character reveals, the quartet aren’t the only people eager to escape.

Sequestered in a death-plagued house for at least the next month are the Snelgraves: an aristocratic couple (Andy Brownstein and Nanna Ingvarsson), a creepy child (Sarah Taurchini) who may or not be their wealthy neighbor, and a pirate-y sailor (Davis Hasty) who sneaks into a house he thought was abandoned. It’s clear there is going to be more to these folks than it seems, and Strain’s effective use of jolting music and horror-movie lighting cues prepares the audience for the downright sinister.

But while what we learn about the characters can be interesting, it’s never really shocking. Mrs. Snelgraves has an interesting back story, but the characters’ reactions to it are predictable. The child Morse always feels to be more of an enigma than the script fleshes her out to be, and Taurchini does a fine job capturing the character’s eeriness. Mr. Snelgraves is probably the most nuanced character of the bunch, and Brownstein paints a complicated portrait of the character — his scenes with Hasty’s sailor, with whom he is fascinated, have the most unpredictability and tension to them.