The film shares troubled romance and cold war sub-plots in common with the other movies with protagonists Collins (Nancy Everhard) and McBride (Greg Evigan) balancing a relationship alongside their duties as part of a group maintaining an undersea missile base. Sexy science babe Scarpelli (Nia Peeples) voices concern over the planned detonation of some prehistoric underground caverns but arrogant South African scientist Van Gelder (Marius Weyers) blows them up anyway. The explosion unleashes some sort of giant, scaly, three-lipped mutant prawn creature that attacks the base causing no end of problems.
Plotted by numbers in a manner frankly reminiscent of Friday the 13th, Deep Star Six keeps its monster out of action for an inexplicably long stretch while Cunningham, an efficient rather than inspired horror director, singularly fails to build much in the way of suspense. The awkward pace keeps breaking away for a fresh twist (unexpected pregnancy, marriage proposal) in the McBride/Collins romance while the dialogue is half pseudo-scientific gobbledegook (“Have you ever heard of photomigration?”) and soap opera waffle (“Let my baby live!”), but the cast of TV stalwarts (Greg Evigan was in both Seventies trash TV favourite B.J. and the Bear and dodgy Eighties sit-com My Two Dads) deliver appreciably committed performances. Victims are predictable (the nice Russian scientist and good-hearted black captain are the first to die) yet surprisingly few fall prey to the Chris Walas-designed monster whose exact origin remains a mystery the film fails to resolve. Instead most of the fatalities involve malfunctioning equipment (e.g. death by bloody decompression; hatches that chop people in half), accidents and one hilarious incident where panicky idiot Snyder (Miguel Ferrer, doing well as the token sweaty incompetent jerk) accidentally shoots another character with a raft-inflating cartridge.
Despite having a cheaper budget than the other two aquatic sci-fi opuses that year the effects (including input from Greg Nicotero, veteran Jim Danforth and Steve Wang, future director of The Guyver (1991) and Drive (1994)) are decent and the sets suitably claustrophobic with efficient cinematography by Mac Ahlberg, onetime director of European erotica like I, A Woman (1965) and Fanny Hill (1968). The inclusion of three strong female characters proves regrettably deceptive as the monster reduces each of the hitherto smart and capable women to hysterical, blubbering wrecks. In line with Cunningham’s Friday the 13th rules, one seemingly important character pays the price for having sex a few scenes before. Given the script, co-written by Lewis Abernathy and Geoff Miller, makes such a big deal about Collins’ naval background (to the extent that McBride is intimidated by her allegedly badass credentials), one would expect the finale to involve her facing off Ripley-style with the monster, but no. Cunningham recycles his Friday the 13th “gotcha!” with a finale that reinforces the notion of the story arc essentially serving as a boost for McBride’s bruised male ego. Incidentally, count the amount of times Collins tries to save someone only for McBride to hold her back saying “There is nothing you can do.” Yeah right, pal.