It starts with a wide shot of a vast body of water littered with fallen trees and surrounded by ominous mist. Then, The Grim Reaper (Donald Pleasence) appears to warn children about the dangers of behaving foolishly around water, especially when one does not know how to swim. What accompanies its words are three scenes of kids plunging into the water while doing risky aquatic-related activities such as retrieving a football from a pond, fishing from a fragile branch, and going for a swim in a polluted lake. With the implication being that they drowned, and the spirit has come to reap their souls.

Lonely Water, commissioned by the Central Office of Information, is a British Public Information Film scripted by Christine Hemmon and directed by Jeff Grant. Since 1973, and for several years, it was aired in various children's programming as a response to concerning statistics on child mortality rates in drowning accidents across the UK.

It is widely regarded as one of the most terrifying Public Information Films to ever exist, said to have traumatized an entire generation of children.

Tropes:

• Fighting a Shadow: The Spirit of Dark and Lonely Water is presented as a Grim Reaper-esque figure who floats over water and land alike, coming for drowned children's souls. It looks corporeal, but as soon as a girl acts sensibly, the spirit disappears into a heap of robes.
• The Grim Reaper: It features Donald Pleasence dressed up as Death (here called "the spirit of dark and lonely water"), a cloaked figure with shrouded features and a hazy aura. He then proceeds to monologue about the drowned children whom he's here to reap ("the unwary, the showoff, and the foolish").
• In the Hood: The Spirit of Dark and Lonely Water is the Anthropomorphic Personification of drowning. The Spirit wanders near bodies of water, looking like a sinister monk, waiting for foolish, unwary, and arrogant children to fall in.
• Ominous Fog: The Spirit of Dark and Lonely Water is introduced floating over a hazy lake with mist shrouding him. Since it's there to collect drowned children, his presence is very ominous indeed.
• Power Echoes: After he's defeated by a heroic girl and her friend, who rescue a drowning boy while staying safe themselves, the Spirit of Dark and Lonely Water promises to come back in an echoing voice. The Spirit, as the embodiment of drowning, is a preternatural force always on the hunt.
• Rule of Symbolism: When the girl leading the rescue of a drowning boy appears, the Spirit's robes collapse into a heap that she promptly runs over. She demonstrates cautious yet heroic behaviour by helping rescue the other kid by not swimming over to him, but by offering a long stick that he can then cling to. This way, she doesn't expose herself to any major risks, allowing her to defeat the Grim Reaping Spirit.
• Rule of Three: The Grim Reaper states that children will surely drown if they act unwarily (fishing from a floating branch that is not secure), foolishly (ignoring a "Do Not Swim" sign), or arrogantly (retrieving a football from a sleepy bank to show off their 'courage').
• Scare 'Em Straight: It warns that playing around water bodies is only safe if children act cautiously. By, for example, paying heed to danger signs or ensuring they won't fall off where they stand. The only child to not drown is rescued in a safe manner—by extending a stick that he can hold onto—by other children. To hammer it, the Grim Reaper states that he has no power over sensible children.
• Sliding Scale of Like Reality Unless Noted: The short takes place in a semi-realistic world, which is identical to ours (people die by drowning) except for the Spirit of Dark and Lonely Water's existence.
• Soft-Spoken Sadist: The Spirit of Dark and Lonely Water, voiced by Donald Pleasence. With a very kind fatherly (or grandfatherly) voice for the Anthropomorphic Personification of drowning, it waits for foolish children to have an accident.
• Super Drowning Skills: The children who showoff and are unwary enough are unfortunate to drown and splash down into the deep, deep water, especially when one does not know how to swim.
• Surprisingly Happy Ending: After watching two kids drown because of their risky behaviour next to a deep water body, the third child is rescued by a group of kids who, unlike him, don't venture into the polluted lake but extend a long stick for him to grab.
• Target Audience: It was aimed at the 7 to 12-year-old age group in order to scare children into behaving sensibly when near water bodies whose depth was greater than their height.
• The X of Y: The public information film's alternate title is The Spirit of Dark and Lonely Water.
• We Will Meet Again: Before this was popularized by Arnold Schwarzenegger in the Terminator franchise from about a decade prior, The Spirit of Dark and Lonely Water lets out his famous echoing threat before sinking in the water:
  The Spirit of Dark and Lonely Water: I'll be back!