Verdict: 3/5 — A genuinely unnerving chamber piece of cosmic horror that earns its scares through pure claustrophobia, but two hours in one submarine is a long time, and the thin story can’t always fill it. Watch it for the atmosphere; forgive the bloat.
| Director / Writer / Star | Mark Fischbach (Markiplier) — his feature directorial debut |
| Cast | Mark Fischbach, Caroline Kaplan (Ava), Troy Baker, Elsie Lovelock (the Monster), Seán McLoughlin |
| Runtime | 127 minutes |
| Budget | ~$3 million, self-financed |
| Genre | Cosmic horror / sci-fi |
| Bottom line | Superb dread, shaky pacing — a 3-star nerve-shredder |
Plot Summary (Spoiler-Free)
After an event called the Quiet Rapture — every star in the sky simply went out — what’s left of humanity clings to stations and ships. Simon (Fischbach), a convict infamous as “the Butcher” for his role in the destruction of the Filament space station, is offered a deal: pilot a tiny experimental submarine, the SM-13 — the “Iron Lung” — through an ocean of human blood on the desolate moon AT-5, photograph points of interest, and earn his freedom.
The catch, which the film weaponizes mercilessly: the sub has no windows. Simon can see the outside world only through a primitive still camera, a motion tracker, and coordinates called out over the intercom by his handler, Ava. What he finds down there — audio logs from a pilot who supposedly never existed, a wrecked sister-sub, blood seeping through the hull, and something moving in the dark — peels his sanity apart one bulkhead at a time.
What Works
The claustrophobia is elite. Confining the camera (and us) inside a rattling metal coffin for nearly the entire runtime is a formal dare, and it pays off. Every creak of the hull lands like a jump-scare; the sound design alone deserves praise as a co-star.
Markiplier can genuinely act. Carrying 127 minutes almost solo is an absurd ask for a first-time lead, and he delivers — the final act’s physical desperation is raw and committed. Caroline Kaplan is excellent too, doing more with a voice on an intercom than most actors do on screen.
Practical, tactile horror. Thousands of gallons of red liquid, real sets, minimal CGI sheen — the blood ocean feels wet in a way digital effects rarely achieve. The body horror in the third act is memorably nasty.
The cosmic dread lands. This is horror about an actively malevolent universe, not an indifferent one — closer to Lovecraft than to standard sci-fi. The film understands that the scariest thing about the unknown is that it might be looking back.
What Doesn’t
It’s too long. At two hours with one location and essentially one actor, the middle sags. Twenty minutes trimmed would have made this a significantly better film.
The plot is thin. Hallucinations, a giant eye in a red sky, a godly light beneath the ocean, a nebulous monster — the threat keeps shifting shape (giant fish? bright light? voice?) without ever quite cohering. If you need your horror to make sense, this will frustrate you.
The ending underwhelms some viewers. It’s deliberately anticlimactic — bleak rather than cathartic — which is thematically honest but dramatically muted. (Details in the spoiler section below.)
Cast & Performances
- Mark Fischbach (Simon): A real surprise — grounded, physical, believable. The rare YouTuber-to-filmmaker leap that actually sticks the landing performance-wise.
- Caroline Kaplan (Ava): Commanding entirely through voice; her late-film confession recontextualizes everything.
- Elsie Lovelock (the Monster): Angelic one moment, fiendish the next — a genuinely unnerving vocal performance.
- Troy Baker & Seán McLoughlin: Solid support in limited roles.
The Ending (SPOILERS)
Simon learns the truth in layers: he wasn’t the sub’s first pilot (hidden audio logs prove it), Ava has been lying about his chances, and there is no rescue coming. When a massive entity closes in, Simon makes his choice — he overloads the sub and drives it straight into the creature, ensuring it implodes and takes him with it.
He doesn’t survive. But the final image — his life vest floating on the blood ocean, its beacon still blinking — suggests the data he gathered might be found. It’s the film’s thesis in one shot: no triumph, no catharsis, just a costly sliver of hope for a humanity that may already be doomed. Grim, on-brand for cosmic horror, and the most honest ending the story could have.
Where to Watch Iron Lung
Release has rolled out theatrically through 2025 (including international markets). Check local listings — and note the film’s sound design rewards a proper theater with good surround sound far more than a laptop viewing.
Frequently Asked Questions
Is Iron Lung based on a video game?
Yes — on David Szymanski’s 2022 indie horror game of the same name. Markiplier played it in summer 2022 and announced the film adaptation in April 2023.
Did Markiplier really finance Iron Lung himself?
Yes. He wrote, directed, starred in, and self-financed the film for roughly $3 million through Markiplier Studios.
Does Simon survive at the end of Iron Lung?
No. Simon deliberately implodes the submarine to kill the creature, sacrificing himself — though his floating life vest suggests his data may still reach humanity.
How long is Iron Lung?
127 minutes (2 hours 7 minutes).
Is Iron Lung scary?
It’s dread-heavy rather than jump-scare-heavy: claustrophobia, body horror, and cosmic unease. If films like Solaris or Event Horizon work for you, this is your lane.